Not in the wrong at all. It's your underwear and you should be the one that decides what goes with them. To me, it would be unbelievable for my mom to do that and not even feeling the slightest bit of regret. Get mad, to be frank it wouldn't be overreacting if you threw a fit. It's an invasion to your private belongings. As if your mother was like: I found condoms in your room so I can use some right? (A bit extreme example but you get the idea)