Only gained 2lbs... 18 weeks. Worried !!!

I am still nursing my 17 month old at night and I am 18 weeks pregnant. I am starting to get a bit worried. I started off at 132 lbs and now I am 134lbs. I am 5"7. With my first I was at the start 115 lbs and gained a lot of weight fast... now I am not :/ what gives ? I am tracking my calories and am eating 2000-2600 a day.