Everyone's pregnancies are different. If your doctor doesn't seem concerned with your weight gain then you shouldn't be concerned either. If your really worried about it then bring it up to your doctor and ask them for suggestions on keeping your weight gain down some. My first pregnancy I gained a total of 30 pounds. With my second pregnancy I gained about 30 pounds with her. With this one I'm 37 weeks right now and I've gained 35 pounds. I've always been a small person and the weight gain defiantly bothers me but my doctor says I'm right on track so I just try to ignore the number until after baby is here and I can lose the weight.
If its definitely not diet related (and only you'll know the truth about it) then you may have pregnancy edema. Do your rings still fit?
Maybe you have a lot of fluids
Everyone is different. If you're eating healthy I don't see why u have gained so much. But ask your doc. Im 26 weeks only gained 9 and I've always been petite.