Normal to gain this much weight?

So I started this pregnancy at 156 pounds and I'm 5'6". Of course that means I'm considered over weight according to BMI, but I find BMI to be a joke, because all I did was work out before I was pregnant, so I was pretty much all muscle. Anyway, I had severe nausea during the first trimester and dropped down to 148, then once I hit the second trimester I started to gain some weight slowly. Well, the last three weeks I've gained 7 pounds! I'm now at 160... Has anyone else had that dramatic of a weight gain? I just don't know how it's possible. I eat well, and I go for walks. I'm 21 weeks and 4 days.