No period but not pregnant!!

I had been nursing my LO for nine months when I had my first period. I had her completely weaned by 10 months due to my milk drying up. I am now on day 37 of my cycle and still have not had another period. I have had two negative tests because I was concerned. I have always been pretty regular between pregnancies. And have been having cramping like I'm going to start for bout a week. Anyone have any ideas or advice!