You're thinking about this the wrong way, OP.
You didn't get caught masturbating, she interrupted your masturbating!
She's acting like you're in the wrong. Of course you're not! She didn't even have the courtesy to knock, and it's not like you whacked one out in front of everyone. It's a disgrace on her part; if anyone had the audacity to do that to me, I'd make sure I wasn't the one left feeling embarrassed.
Masturbation is used to fulfil a hunger; similar to food, the only people who obsess about it are the clinically obese and the anorexic.
Yes, I did just suggest the reason she wants to guilt trip you is because her sex life sucks.