I think I got cheated on
What makes you think that?
I found a pair of panties in my boyfriends car
It is wrong for your boyfriend to cheat on you. You should trust your suspicions and confront him, and tell him that people are supposed to be faithful.
I'm going to turn my suspicions into actions and make him feel bad.
Remember that there is at least a small chance that this is not evidence of cheating, so don't be too quick to confront your boyfriend (he could have let someone use his car to change, or to transport old clothes to the trash) of infidelity.  However, if he doesn't have an explanation, you should definitely let him know that you expect him to be loyal and faithful.