Hi Vita,
Dont worry about it my weight gain has been really erratic, at first I bloated and but on a few pounds in just water or something, but then as sickness kicked in I found I could no longer go without breakfast and I couldnt eat huge meals either, so I had to eat small meals but more of them, and although I could see I was gaining a tummy, I wasnt puting on any weight and actually lost a couple of pounds. Im now 5 months and over the last 3 weeks have been chucking on the weight. I dont think in the first trimester you really put on much weight as the baby is more developing its organs and limbs etc, it only really starts growing in the second trimester, by which time the average is to put on 1lb a week.
So dont worry about your weight it sounds about normal.