Lack of weight gain

I'm 21 weeks and 5 days today, and I still have only gained four pounds. I eat plenty so I'm not sure where it's all going? I have hypothyroidism and I take 50 mg synthroid everyday and get checked by a specialist once a month. So far baby measures healthy, but I'm nervous. Anyone else not gaining weight?? Should I do anything? (Photos to show my build and the small bump I have)