Gaining too much!

I need some love and encouragement. I saw my midwife today for my 18 week appointment and I am up about 15 pounds!!! Gahh! I can't handle this! It's my 3 rd pregnancy and I eat healthier and am fitter and more active than I ever was the first 2 times! Now my midwife wants me to watch my calorie intake! I feel like I am doing something wrong! Am I the only one who gains lots of weight without trying? I don't eat much more that I did before.