What Causes Irregular Periods, Dizziness And HPT Negative?

I have only had two periods since the birth of my second child one in Feb2010 and the other Oct2010 over the last 3 weeks i have been feeling increasingly sicker throughout each morning and keep going dizzy i have done 4 home pregnancy tests all negative results so im guessing that is not the problem i went to my doctor who was not that concerned please help me i need to know what is wrong