What is the difference between a fat belly and a pregnant belly?

I still get my "period" and all tests said no. If I was I would be 29 weeks today, and I've looked up pregnant bellies at this stage and they are much bigger than me. But my stomach is growing and growing. And I'm not overweight at all. My diet has not changed. I measured around my stomach and it has grown two inches in the past couple days. My stomach isn't a "fat roll" though. It is perfectly round and firm (but not rock hard). I seriously look pregnant and I am very limited to what I can wear now (so that I don't look like I am pregnant).

I don't know what to do. I would feel so embarrassed if I went to the doctor and they are like, "no you're just fat".