What Is Causing My Irregular Menstruation With Dizziness And Nausea After Child Birth?

I'm 23 years old and had my son in December 2009. My period returned in March this year and has been regular - until July. In July, it was on time, but really short. In August, it was a few days off and really streaky and lasted only a day. Now, September, I have yet to see my period. I've done 4 at home pregnancy tests a week apart and they're all negative. I'm getting really bad cramps like I'm going to have my period, but no bleeding. To add to that, I've been dizzy, nauseous and I've noticed a dive in my appetite. I can't be pregnant though, right?