Weight gain on the p90x program?!

I started the p90x program about 4 weeks ago, and am now on the 4th round of the classic version. My question is, when I started this, I was 102, I'm not doing p90x to lose weight, but to get stronger/fitter. But since I've started I've gained 10 pounds. I don't think this is fat, since my body fat percentage has gone down, and though my clothes do feel a bit tighter, its only been very recently that they feel just slightly tighter. So basically they fit almost the same. I didn't think it was possible to gain that much muscle in just 4 weeks (10 pounds worth) but I don't know whats going on and t he numbers are kind of throwing me off. Is this normal? should I expect more weight gain? I don't know if I'll be able to handle that, I'm already ready to quite because of the weight gain! I know that 112 isn't that bad, and is fairly thin, but I'm short (5'3") so it makes a difference and it's just frustrating seeing the number go up when I've been very very healthy.