What Causes Abdominal Pain During Periods?

for a few years now, i have upper abdominal pain that comes on only when i get my period. for 2-3 days i feel like i just had stomach surgery. it hurts to walk, firtst sit down and eating makes it worse. no medication helps. just have to suffer. could i be allergic to tampons? endometriosis has been ruled out, no doctor can figure it out? deperate for an answer?