It might just be that your uterus has popped over your pubic bone which is making your tummy seem huge. It's about this stage of pg when your uterus is too big to remain entirely in your pelvis.
Don't worry too much about your weight gain. If you haven't gain alot yet, then maybe your body is catching up a little. If you haven't been eating anything different, then it's likely to just be 'baby' related weight. For me I don't gain alot when pg, but I am/was overweight to begin with, so it might have been different had I not been heavy. But there were still some weeks I would gain a kilo or two and others I wouldn't gain, or even lose weight. I'm sure if your weight gain is of concern, your doctor will let you know.