Am I pregnant? I'm a little worried.?

I don't think I am but I'm still worried. I'm on birth control (Loestrin 24 Fe) and I always get my period at the end of the month. Even if it's just a little bit. But I haven't. It's been a week and I took a pregnancy test two days ago and it came out negative. As for other things...I just drove to Hollywood, not much of an elevation change. And I recently stopped drinking soda so I am feeling really sick due to caffeine withdrawls. Oh and the week before I was supposed to get my period I was very stressed due to midterms. But I don't think those things would have much of an impact on my period. Should I see my doctor? Maybe get a blood test? I mean I usually have very regular periods so this is pretty odd.