Question concerning weight gain

I'm in my second trimester now, and haven't gained any weight. I've lost about 6 lbs since my starting pregnant weight. I started my pregnancy a little overweight already, so I know I don't need to gain very much weight at all, maybe 10-15 lbs per my doctor.

My question is, will I gain this weight in the third trimester naturally, or should I be making an effort to increase my calorie intake now? I don't want to gain a bunch of unnecessary weight, but I also don't want to put the baby at risk of not getting the calories it needs.