What Could Menses Cycle With Post Delivery With Extreme Headache Suggest?

Hi, I recently gave birth in October through IVF and shortly thereafter began having my menstrual cycle again beginning in December. The last time I had my cycle was January 8th and am like clockwork when it comes to being on time. I took a pregnancy test on Monday before my cycle should have started and it came back with a negative results. I have been extremely tired and have had a constant headache for the last 5 days.