I feel wrong for having sex

I'm 18, and i've been sexually active for 4 months. I was raised religiously and that pre-marital sex was wrong, but I never felt that way and never felt guilt. That is, until recently. Every time my boyfriend tries to start something, I start to feel sick and I can't do it. I don't think I ever had a real change of heart or anything. What's wrong with me? 😞