Hi. Thanks for your query. Pains in the abdomen after a day in the gym definitely suggests the muscular cause. Due to the already ailing muscles being subjected to the strains and sprains of the exercises, it never gets rests enough to regains its normalcy. Hence this is something like a repetitive stress injury of that particular part of the muscles which when strained gives you menstrual like cramps. But, to be sage you have to undergo an ultrasonography of the abdomen and the abdominal wall just to see there are no other causes like ovarian cyst getting twisted or PID getting exacerbated and so on. I would suggest you to be seen by a Surgeon for physical examination of the abdominal wall for the muscular problem or a hernia and to get an order for ultrasonography and a prescription for the anti-inflammatory medicine with a painkiller property. No exercises till you get alright.