Is diarrhea a sign of pregnancy...or the pill?

I know I must seem ignorant but I'm just wondering what it could be. I've had nausea since starting my birth control pills this month but I also had sex on Valentine's Day when I hadn't had a chance to get my pills yet. Though it was protected and I'm probably just getting inside my own head, I'd like to know about my symptoms. (Nausea, Headache, Diarrhea)