How do you know?

So it's been a week since I last had sex without a condom. I am on the pill though, but the day that we had sex was one of my placebo days and the next I started my new pack, but changed the time of taking the pill in the day. Lately I have been feeling weird little cramps and I think you are suppose to wait 2 weeks after sex to take a test. I was just wondering if this was an early pregnancy sign? Also which test is best to take?