Do you think she cheated?

last night we had a party and we got in a fight about 3am, everyone ped out in the livingroom and the back room, my gf was in the living room with 2 other people and another guy was in the back room who she had been talking to all night,i went upstairs to bed but couldnt sleep, i heard something about 20 minutes latter and got up but when i got to where i could see down where my gf was laying i saw her quickly getin down and the burry her face in the pillow, i came down and asked her to come upstairs again and she refused, i went and checked on the guy in the back and he was in the bathroom with the water running for like 5 minutes then he came out with sweatpants on. Then i went in the bathroom and found a hand towel with when i think is some m on it and it smells like my gf's in one spot. We live to gether for 2 years in a house, not married, dating 3 years, never cheated on me before, I don't know if i should say something to her and regret accusing her of something she didnt do or what to do.