Weight gain question

Hi everyone! I am classified as obese and was on Noom prior to pregnancy and lost 20lbs. When I found out I was pregnant I still logged my food but bumped up my daily calories from 1350 to 1600-1700. But I’ve gained about 5lbs in the first two weeks (3weeks 4 days to 5 weeks). Any ideas on why this might be happening? I am drinking a lot more water but also getting the recommended 150 minutes of moderate exercise. I’m nervous because since I’m already obese, I don’t want to harm the baby. I’m just confused why the weight would pile on so fast. Any help, insight, or advice is greatly appreciated!