For Upto The Past Year I Have Been Having A

For upto the past year I have been having a pain on my left side just under my ribs. The pain is horrendous and no over the counter pain killers help. It reduces me to tears and I have to contort my body into funny shapes to illuviate the pain. This occurs a few days before I start my period and usually a night just before Im going to sleep. I'm only 18yrs old. Is this anything serious as it doesn't sound normal?