Undefendable black pawn C4

Why is this undefendable?

I know i'm not very good, obviously the top players can run rings around it, but i've just defended it easily in a game on here. In fact, i usually do defend it and stay a pawn up the entire game. My opponent was only about 1290, but I've even defended it on Fritz as high as 2800. So why is it undefendable and are players too eager to lose a pawn thinking it's undefendable?

How do i post the game i just played to explain what i mean better?

It may have been 2400 on Fritz. Or 2600. I can't remember now. I've lost my database after switching computers. Backed it up somewhere but can't find it!

If we told you the answer, would you remember it?

Probably not!

To be fair, the games i mean were months ago. Not played Fritz in ages, don't wanna play it til i restore my database. I know it's somewhere, but i have about 8 USB hard drives full of crap!

Book yourself on a data housekeeping course immediately

Never a truer word spoken. I admit it, i'm hopeless with it. I never know where anything is. Always recording stuff off the tv, then just dumping it all on a hard drive without sorting it all out. My photos and files are everywhere too. It's a bit of a nightmare!