Am I pregnant?

So I should have started my period exactly two days ago, and I have been having cramps as if I was on my period. But these cramps were a little bit worse. I haven't been spotting at all. I did have unprotected sex a few weeks ago with my boyfriend and used the "withdrawal" method a few times. Although I have been under a lot of emotional stress these past few days, my periods have always been fairly regular. Always exactly 25 days apart. I've been taking the hcg pregnancy test strips but they have all been negative. Could I be pregnant or is it just stress? Any input is appreciated :)