What is causing my sudden abdominal pain? I had a lot of popcorn.

Could be. Diverticulitis or just severe constipation. People eating popcorn in large quantities often don't drink enough fluid. That being said, if you have a fever with this problem you must assume you have diverticulitis and need lab work, an exam, possibly a ct sscan of abdomen and pelvis and 2 weeks of antibiotics. Don't delay. This could be very serious!
Possibly. But no way of telling without thorough history and physical. Go see a doctor if it gets worse or persists. Could be one of dozens of things.