Lol this app has told me that my weight gain is excessive! I had a healthy bmi before and I've gained 22lb at 34 weeks so on track for the normal pregnancy weight gain of 25-35 lb. So my weight gain is in no way excessive as this app says. I wouldn't worry as long as your docs say your fine then you will be.
I was also underweight for the first half of my pregnancy and now considered overweight according to the app. My OB is happy with my weight gain, so I'm ignoring the app's weight tracker. It is not at all accurate nor reliable.
Just ignore it. I hate the weight tracker on this app just for that reason, it's never a good idea to tell a pregnant lady she's had "excessive weight gain"... Ugh. But just ignore it and so long as your doctor isn't concerned I wouldn't worry.