I was at the high end of "normal" for weight/bmi for my height but am already worried that I am gaining too much

I wanted to gain 10 lbs at 20 weeks and I have already gained 16 lbs at 18 weeks....uggg. I started exercising again (moderately - 30 min. walking, 4-5 times a week) and am trying to make healthy choices but I must be eating too many calories. I didn't want to count calories during my pregnancy but I think I might have to.