I haven't had an full period since sept 27. none for oct. nov some bleed which was nov 2-5th. belly gets really big when i get fully what is this?

Start with test: If you are sexually active the first thing to rule out is pregnancy so do a pregnancy test. There are a lot of other things that can cause this. Some are GYN in nature and others are GI so, if the test is negative, see your primary care provider for a full history and exam to decide which tests are necessary for a diagnosis. Feel better soon.