Could I be Pregnant?

I am four months postpartum With my wonderful son. He only breastfeeds about once a day and to fall asleep at night and it's been like that for about a month. I still haven't had a period. For the past few weeks I've had mild pregnancy symptoms: frequent urination, cravings, extreme fatigue, very emotional. I've taken several pregnancy tests and they were all negative. What the heck is going on?