When i exercise or sit down in have a really big bulge in the middle of my stomach. i also haven't had my period in 3 month. i exercise quite a lot ?

Several causes: The abdominal bulge could be a sign of pregnancy, a hernia, or a tumor. With the fact that you haven't had a period in 3 months, I'd recommend starting with a pregnancy test if you have been having unprotected intercourse. Other things can cause the delayed period and you will need to see a doctor to either care for the pregnancy or look into the cause of your symptoms. Hope this helps.