Abdomen Pain, Gas. Is This A Gynae Or Gastro Problem?

Hi, I ve been having pain mostly in the lower left of my abdomen for about 6 months now, it all started with bad lower back ache and have had 4 periods about 2 weeks apart. Doctors don t seem concerned, however I can feel a mass in the same area, not a lump, just an area of fullness. I used to exercise a lot but haven t done any since this started and tonight did some jumping jacks, I could feel the pain there when I jumped. The rest of the time it s a dull aching pain, sharp pain occasionally. I m suffering from a lot of gas and rumblings seem to occur in the same area also, in fact my bowels feel like they are in a state, to the point it s stopping me wanting to go out, one minute urgent need to go the bathroom, next minute I can t go without laxatives. I m really not sure if this is a gynae or gastro problem. I m a 40 year female.