My girlfriend thinks i had with her best friend last night?

I work with her and sometimes i get home late. Last night as soon as i got in my girlfriend began allthe accusations and insisted i let her *see* to make sure i hadn't had . I was absolutely mortified and i refused, not that it would prove anything. Because i didn't let her touch me she thinks it's proof that i did. Please tell me, is she for real or is she mad and what should i do?