it must be confusing to feel pg and have neg tests. i suggest going to see your dr. there are several things that could cause your stomach to swell and a false neg could be one.
Unusual weight gain is a reason to see the doc anyway. Don't be ashamed to go for that symptom. Believe me, they've seen it all.
You will not gain weight in early pregnancy. If you have any symptoms and have missed period that's the first sign of pregnancy. I suggest visit doc ASAP. They will tell you in what situation you are. All the best.
I agree with the others, see your Dr. That's alot to gain so fast, maybe something else is going on, I never gained that quickly with pregnancy, but we are all different. Good luck.