How can this pawn move be a blunder?

Hi fellow chess players! I need your help. I'm not asking for a full analysis of my game, I'm only asking about one specific move. The game went like shown below. 

How can this pawn move turn the tables so drastically? Is the Computer Analysis here on Chess.com really trustworthy? If yes, what am I not getting? Thanks!

It allows white to lock up the king side and thus stop all of black's threats.

I appreciate that you are answering my question. But it's rather vague, and still it seems as if this reason cannot explain how this one move alters the score with more than 4(!) pawns. Could you elaborate? Does anyone else dare an explanation?

h3 and black's got nowhere to go with his initiative.

True. But does that explain a change of score of more than 4 pawns? I don't see how.

Well, one of the reasons your advantage was so great was because f3 looks like it loses a piece or allows black to threaten to queen, whereas g3 does nothing.

I see. But 7...f3 doesn't win a piece, does it? I only see it winning a piece for two pawns. Isn't that correct?

I'm not sure, I didn't do too many calculations, but it does looks to chase the white out of its hiding.

I lied. Nxf3 gxf3 Bxf3 and white seems to be doing fine in terms of immediate threats. Still, being down a piece is always unpleasant. I'd put the evaluation at least two pawns in black's favor, seeing as before the exchanges he was already up a pawn. A pawn for a piece, in the opening/middlegame does seem winning, especially with equal development where neither side has really much of an initiative for an attack.

I believe a number of people have found the computer analysis here to be not exactly trustworthy. Do a search on the forums here on it and you'll find some discussions on it. You're better off using something like Houdini I think, although the computer here is useable...

But yeah I'd assume dracoms is right on why your score went down.

Regardless of the analysis, I can almost guarantee I could lose from that position.

Anyway, 4.d3 was bad.

I don't know how the comp calculated it but the attacking opportunities for Black are tremendous in that kind of lines where White plays careless.