Is My girl cheating on me?

my girl is always bitching about something. that hasnt changed and i know that she means good by it but it is steadily wearing me down. recently she started saying she wants less often cause she is sore. in the last 13 days we have went through 16 condoms right but last week she made me go like 4 days with out nothing now shes pulling the same bull. but thats not what made me start thinking about it actually her and my friend said some things that made me wonder? first i just called to see where she was. we were all drinking and she had just walked off then when i called her she was like iam getting more drink with a friend, well i got mad that she was giving away my drink and and the guy jumps on the phone and says we aint doin nothing. then latter on iam talking to him and hes like its a good thing were good friends cause if you were any one else you would think me and your girl were doing something. then tonight when i ask to mess around she trips on me and i get mad and leave ?