Is my girlfriend cheating? ?

We r coming up on 9 years and our sex life has plummeted. Before we used to be like rabbits could it just b we r tired of eachother after so long or did she find someone else? I approach her a few times a week and I get the I'm tired or got a headache excuse the only time she approaches me Is when she is drunk and of course I say no in spite of the times I get shut down. So women please help me see the other side do u think she is or is there another reason im not seeing? Thanks for ur help.