Why are my period cramps so bad?

On my first day, i can't move, the pain is so bad i have to lay still with a heating pad, and walking is out of the question. Midol, pamprin etc.. Don't work, I've tried advil, aleve, asprin, etc... Also dont work, not even the loratabs i had from getting my wisdom teeth out worked. i'm not over weight, i eat normal, or out of shape.. i don't even bleed very much. I don't understand why they're so bad, my mom said hers never hurt and my sisters don't either.. Why mine? Can this effect my fertility? I've unprotected sex over a year now and yet to fall pregnant, so i'm begenning to wonder! Thanks, for any information!..