Holy weight gain! 7 lbs in one month!???

I was overweight to begin with so I'm not supposed to gain much. I had been doing so much better than I had in previous pregnancies. At my 16 week checkup I was up 4 pounds and my doc was happy. Since my last checkup however I've gained an additional 7 pounds! I'm so stressed and upset. I know I'm going to be yelled at at my appointment on Wednesday. I don't understand it. I've been eating well and walking an hour each night. So here I am at 20 weeks and I've aready gained the max I'm supposed to gain :(