Sex drive

Lately I haven't been wanting to have sex, usually I'm too tired.. but now my boyfriend says that its weird that my sex drive has been low lately because it's always high and I know now that he thinks the reason my sex drive is low is that he thinks I've been sleeping with other men. I haven't. About a year ago I did kiss another guy while we were dating and it was the worst thing I've ever done. I felt so horrible cuz we were already dating for 3 months and I go and do that, but a year later after not doing anything like that again he still doesn't trust me which I can somewhat understand but thinking that sleeping around and then refusing to admit that that's what hes thinking?? He's not even giving me a chance! I tried to explain to him that sometimes a girl's sex drive fluctuates plus I just got put on birth control so that could be another reason. He just won't listen. Could someone back me up?