I think my girlfriend is cheating on me can anyone help?

There are a lot of things that lead me to believe this she changed we were happy the first couple months she was nice to me attracted to me we had plenty of once to three times a week used to make out a lot used to cuddle and watch movies we used to be very intimate and do a lot of pda now she treats me like **** she uses me and won't cuddle or kiss and has gone down a lot its been over two weeks and not a dam thing not even some kissing lasting longer then 30 seconds before that, it was over 3 weeks. we haven't made out in like 3 months I have to force her to kiss me when shes with me and is talking to her friends she never mentions that she is with me she always says I'm eating dinner or I'm watching a movie everytime I want to have with her she always has an excuse not to I have to like force her to eat her out god forbid she give oral I like have to force her to kiss me and she'll kiss like here you now leave me alone she's already said I'm cheating on her she used to hold my hand every where we went we haven't done that in a long *** time I mentioned that I think she's cheating ok me to her last night and nothing she barely said anything and told me good night I love you and went to sleep do you think I'm right what should I do to make sure of it can someone help me???