What To Do For The Weight Gain After Giving Birth?

I have never had a problem with my weight until after the birth of my first child. I have three children - all c-sections. I only gained 24 lbs with the first, 18 with the second and 12 with my third. Each time I lost the weight immediately and kept it off. When each of my kids were about 4 - 4 1/2 months old i began gaining weight and can't seem to lose it. I have also been suffering from diarrhea on a daily basis - after almost every meal but still continue to gain weight. When this problem first started about 6 years ago I had started drinking a lot of bottled water????? I do have a thyroid condition and am under the care of a doctor and my blood work comes back normal. My upper abdomen is firm and protrudes. I look pregnant but am not. What could be wrong with me? Could you offer some suggestions that I may talk with my doctor about or a specialist?