Weight & Pregnancy

So I had my 32 week appointment today, everything was good. Belly measuring well, both our hearts are strong and good but now my doctor is starting to get a little concerned cause I've been trying to gain weight and haven't in 12 weeks. Now my pre pregnancy weight wasn't small, I want to say I was around 155 maybe 160 and for the last 12 weeks I've been stuck at 178. She's had me schedule a sono in 2 weeks just to make sure everything is fine and I'm sure it is but has anyone else had this happen? I'm a little nervous about it...I know it's nothing scary and not gaining weight isn't always a bad thing.