I'm confused

My last period was November 12th. I'm regular. I've gone to clinics and rook those free pregnancy exams they offer. I want once in mid January and another time at the end of January. I'm going again this Wednesday. They've all been negative. I have no pregnancy symptoms. Before my period i get slight back pain, pulse on my right ovary, and boobs get sore. Ive experiences that Dec, Jan, & right now Feb But no period! Could yall help me understand what could be wrong :( I have been somewhat stressed...