Weight gain stopped

I'm 21 weeks and haven't gained weight in over 2 weeks. Admittedly, I was obese pre pregnancy. I've gained 7.6lbs in all of pregnancy, and all in the 2nd trimester. However the past two weeks, almost 3, nothing. And it's not for lack of eating. The doctor would like to keep me between 15 and 20lbs of gain total, and I've heard lots of people stop gaining in the 3rd trimester, but not the 2nd. I go back to the doctor on the 12th. At what point should I be worried?