Your problem was 20...bxa6. If instead you had played 20...Nd4 you would have attaked his Q and defended your b7 pawn with your own Q.
The best he can achieve after that is an exchange of queens and your huge material advantage will give you the win.
I need help on this game.What move made me lose the game??
I'd suggest overconfidence based on how I played the 75% of the game leading careless play. =)
Basically it was a 5 10 game I'm ranked like ~1130 I think you over 1200.
3 moves in a mate turns up and starts talking to me about some important stuff so for the first 15 moves I was trying to play and talk to him then hes like getting annoyed with my slow response time so I'm like come back in 5 min plz when this game is over. So he goes...
I'm thinking I'm totally stuffed but may as well play it out and see what damage I can do and to see how he gets me... until like move 19 when I see the corner and I'm like maybe one last crack here... I was thinking if he don't take my A pawn I'll take his and he'll be in check then who knows maybe I can get a queen (I couldn't work out how I'd keep it or anything as these where just passing thoughts...). I only had 40s left at this point and he had like 5 min.
Then thinking if he don't take it I have a safe spot on a8 and that's what happened...
I kinda surprised myself when checkmate popped up... I knew it was a very good move that would give him a lot of troubles.... but had overlooked his rook on d7 and didn't expect it to be full checkmate.
For this reason I think this thread title should be "Have you ever checkmated someone when you didn't expect to with that move" or something like that.
That's the problem with games online is the other person cant see the conditions the other player is trying to play under. I'm sure if we were both in the same room you judge would be telling my mate to be quite as well. 
To play decent chess, you simply can't drop pieces. You're not examining your opponents responses.
Up to move 5, good. You have developed three pieces sensibly and have a solid pawn presence in the center. Further, you have an annoying pin on a knight. Good!
5. ... Qe7 looks fishy to me. It's too early. It threatens nothing and barely develops the Q. It hems in the black-squared B.
Examining the board, what can you do? Possibilities include:
[6. Bxc6, doubling his pawns - (but for this to work well, you'd have to follow it by dxe5 - not a bad move, but if he doesn't take back, you don't isolate the pawns...),
6. d5, threatening to win the pinned N - (however 6. ... a6 7. Ba4 b5 8. Bb3 and the N escapes. He'll probably play c6 and remove your d5 pawn, or erode it by eventually playing f5. I don't see a long-term possibility of keeping that pawn there...),
6. dxe5, threatening a N and e6 pawn - (he recaptures and nothing much exciting happens),
6. Bg5, pinning his other N and developing a piece - (This has the lovely threat of the embarassing Nd5, punishing his Q for moving too early and eyeing Nxc7, forking his K and R. Further, this is with tempo, so attempts to kick your pinning B's can be ignored briefly.
6. Nd5, hitting his Q - (but what about his pesky N... 7. ... Nxd5 8. exd5 (threatening the pinned N) exd4+ 9. Ugh don't play this, )
6. O-O - (gets your K off the same file as his Q, and it makes good sense anyway. But it is passive... Except we can then plan for Re1, perhaps tempoing his Q later on.)
]
(There are other moves that I don't like as much, I'll leave you to find those 
)
So, which move will we take? The very fact there are 5 or 6 reasonable moves means black isn't doing so hot.
I like Bg5 and O-O. Before we go any farther, let's check to see if we're hanging any center pawns (my specialty, sadly...) Ok, beware of the tactics from exd4. He gets a boost from his Q placement. For those to work, however, his f6 N must be mobile. While O-O is sensible, I'd select 6. Bg5 and see what he's got.
6. Bg5 Be6 (to keep your N out?) 7. d5 winning a piece.
6. ... h6 7. Nd5 Qd8 (Qe6? Nxc7+ for a K-Q-R "royal" fork) 8. Nxf6+ gxf6 (ruining his castle but opening a file, consider this...) 9. Bh4 maintaining the pin on f6.
Now white threatens to win the e5 pawn with some really ugly variations for black if he doesn't surrender it willingly (ex: 10. dxe5 dxe5 11. Qxd8+ Kxd8 12. Bxf6+ winning the h8 rook.)
Another possibility is
6. ... a6 7. Nd5 Qd8 8. Bxc6+ (so the B doesn't hang after more play elsewhere) bxc6 9. Nxf6+ gxf6 10. Bh4 with similar pressure on e5.
I don't know if that's the best plan, but it's fun.
So, on move 6 I'd have done something different. 
(EDIT - ok, just played through the whole game... and white WON. Well, aren't I embarassed 
Everything above stands. However, let me look again...)
You were killed by carelessness.
You didn't develop your B and h8 R, nullifying your tremendous material advantage. Develop! Develop!
Queenside castling at that moment allowed him to get your 'a' pawn for free creating the situation that allowed the possibility of the kill.
His moves starting with a6 are a series of desperation cheapshots. None of it is sound. Instead, consider 19. Bxd7+ Nxd7 20. Rd1 and he's got threats that aren't easy to stop. In fact, I don't think you can save the N without losing your Q for a R. Black holds with 20. ... Bc5! 21. Qxd7+ (Rxd7? [looking for a discovered check] Qxf2+ winning) Kb8 and white is out of pieces: 22. Qd8+ Rxd8 23. Rxd8+ Nc8. So it's 22. Qd2 to prevent black's 22. ... Qxf2+. The queens soon come off and it's over.
Back to the game as played, it all crashes down on him with 20. ...Nd4! This tempos his Q while allowing your Q to protect b6. If he gets sneaky with 21. a7 he'll find that Nxf3 is check, tempoing his K. Now he's lost his Q for a N and you pick up his pawn with Qa2.
Thereafter, you're up 3 pieces, a rook, and a pawn. His Q and R and disjointed. You can win quickly. If you're careful.
I need help on this game.What move made me lose the game??