Girlfriend Won't Have Sex With Me

I've been dating my s.o for 8yrs. Recently,well within the year she has been turning me down and. Started request me wear condoms and if I don't get it she refuses sex. I think it's weird. We have been intimate the whole time and she refuses to give me oral sex these days. I swear she cheating but she says I'm cheating even tho I come on to her daily. She turns me away. I admit I haven't always been good but neither has she.  I cheated last year but I want to be with her. I know she talked about marriage no kids til we are married but I think I should at least have sex