My girlfriend think I cheated on her. HELP!?

my girlfriend of 5 years is in Florida with a friend for the week on vacation. a few friends and I went out to a bar for a birthday gathering. I got extremely drunk. didn't do anything wrong but rode home with a friend and another friend stayed trying to hook up with a girl. I went home and passed out on the couch. later that night, my friend that stayed at the bar, brought 2 girls back to my house. they woke me up from the noise and i noticed it was 2 sluts that my girlfriend despises. I kinda bitched out my friend for bring them back and then walked to my bedroom and passed out. he sent them home and the night was over. i woke up wanting to tell my girlfriend about the situation but i knew she would get pissed so i was going to wait until she got home. well she found out from pictures being took at my house on facebook and thinks i had sex with the sluts. I DIDNT DO ANYTHING WITH THEM. i was drunk and sleeping while my buddy was trying to get some. i have cheated on my girlfriend a couple years ago so she doesn't trust me. I do not know what to do, I recently purchased an engagement ring for her i just wanted to see how this trip when before i purposed. i love her to death and i want to spend the rest of my life with her. i would never do anything to hurt her again i learned my lesson but she thinks i had sex with a slut. It does look like i did do something because i told her i was extremely drunk. she wont answer the phone and i'm scared of retaliation. please help! what do i do? her friend she is in flordia with doesnt like me and wants us to break up anyway..