Hi welcome to Health care magic forum.
Thanks for calling H.C.M.F.
According to your description, it appears to be the boil with blood oozing in, due to pressure it is causing pain in the stomach. The period has nothing to do with the pain.
As you describe it may burst in about 3 or 4 days, any how you will be seeing your doctor tomorrow He will examine and may draine the abscess
If at all it bursts in the mean time , you keep a wet cloth ,or cotton and tie it with a cloth,to prevent spilling.
Wishing for a quick recovery.