oh honey 15 pounds really is not too bad. I believe that I was right around there when I was at 22-23 weeks! No big deal. I ended up gaining 42 pounds in my pregnancy but in my personal oppinion and my DH I look better than ever. Dont worry its all worth it.
You're not allowed to lose weight during pregnancy. You can, however, do exercises to keep muscle tone. I think 15 lbs sounds okay. They want you to gain about 25-30 lbs. At the halfway point you weigh 15 so hopefully you only gain about 15 more over the next half!