You're singing my song. I weighed 160 when I got pregnant the first time, and was a good 200 pounds when I gave birth. I only lost 20 of the baby-weight and am now pregnant again at 185 pounds! Ick!
So far, here is what I have noticed: my clothes are getting tighter faster. However, I've been keeping track of my weight, and like you, it's not gaining that fast. From what I have read, women often start to show earlier on the second pregnancy than they did during the first because their ab muscles are not what they were for the first pregnancy. So feeling that your pants are tight is perfectly normal.
Secondly, I have joined a gym and am doing low-impact workouts. I am also signing up for a prenatal swim class. Look around in your area and see if there are any pregnancy-oriented fitness options around.
Finally, I am watching what I eat a lot more carefully this time around. Right now, I don't need to eat for two, I just need to eat well. When I get the urge to overeat, I make sure I'm eating raw veggies and fruits instead of the ice cream I really sort of want. ^_^ I don't know if this will hold out indefinitely, but I think at least until the 3rd trimester, my baby really needs nutrients more than anything. My OB agrees, and states that because I am overweight, I do not need to gain as much this pregnancy to healthily support this baby so long as I am eating a balanced diet.
I wish you the best of luck.