Try not to worry, I've said in various threads that having gone through to this point in pregnancy, weight gain is unpredictable in pregnancy! It's not all fat, in fact most isn't. It's baby, amniotic fluid, extra blood, the placenta, general water gain. I lost weight in the first trimester but absoloutely packed it on in the second. I'd gained 24 lbs by week 25/26 and I was really suprised! Bar Christmas, I'd eaten pretty healthily! I got upset about it but then realised that there is no point. I know I'm eating sensibly and not for two. My size 10-12 maternity clothes all still fit comfortably so I'm not gaining much size overall. Now I refuse to weigh myself and go on my clothes fitting and overall appearance.
It's 9 months of your life, you can loose the weight afterwards when you are not growing a small human. Until then, so long as you know you are eating well and being healthy, then that's the most important thing in my opinion.
Oh and in the 3rd tri, for me, the gain seems to have slowed. My stomach feels so squashed I can't eat half as much these days! And looking at myself, my stomach is expanding but not much else!