I am size 2/4 normally. With my first I didn't show till late 5th month. With subsequent pregnancies I've started showing before 12 weeks. I ALWAYS gain weight early. I'm not sure why but it's my body's way. It stinks but I guess it could be worse. Many women come out of 1st trimester same or even down due to m/s. I usually gain 58lbs just in first tri. But I still usually end up at the 3540lb mark and have thus far been able to lose it all and more each time without having to purposely diet/excercise.
This is me exactly, expect I gain a ridiculous amount of weight! I gained 60 with my first son and 45 with my second. I always gain lots in the first trimester, despite eating healthy and being active. I don't get it, but it's just the way it is.
Normally, I'm quite thin, about 6'2 and about 160 lbs. I don't get why I gain so much weight, but i've accepted it. I've lost it all very easily with both my kids and am hoping that I will be able to do that again. I'm trying to be very careful about my weight but at 7w5d, I'm already up 3 lbs and I thought that was pretty good actually! LOL.
Anyway, I wouldn't worry about it. Sounds like you're being healthy! Your doctor will tell you if he or she is concerned :