light period lasting 3 weeks

I'm 22. On 4-13-12 I started my period, for the first two days it was heavy which is normal for me, then it seemed to have stopped, and I only had spotting when I wiped, since then I alternate between spotting and not heavy but enough blood to require a feminine product. Now it is 5-4-12 three weeks later and I'm still bleeding. I haven't had any cramps, which typically accompany my periods, just a constant presence of blood. I've been more tired than usual, my appetite has changed and I occasionally feel nauseous from about noon to two. Is there any possibility I could be pregnant? Could this be something bad? Should I just wait it out?