It can easily be due to your birth control that you are experiencing symptoms. Though if you think you may be pregnant, you can take a home pregnancy test 2 weeks after having sex and get an accurate result. Symptoms are really unreliable, as I get "pregnancy symptoms" almost every month. The symptoms for "pregnancy" are also the same for stress, hormonal fluctuations, medication side effects, illness, and so on.
The thing is, I'm not stressed, at all. So I doubt that's messing with my body. I've been on the pill since august last year (2010) and I've never had something like this. But I just need to wait till next week. I'll keep you updated sweetpea!