Up until about a week and a half ago, I had only gained 3 lbs and hoped that I could keep this slow and steady weight gain going (based upon BMI estimation I am supposed to gain between 15-25 lbs). Then, the last week and a half, weight started showing up. At first I hoped it was just because I had not pooped in a few days, but after fiber supplements and lots of fresh veggies... the weight keeps piling on! I have now added 6 more lbs.

I have a dr. appt on Friday and am scared that he's going to say I'm gaining too much, but I haven't really changed my eating or exercising habits. Is the weight just suddenly catching up to me?
Re: Sudden weight gain
This. It all works out (and can come off) in the end!!
The same thing happened to me around 20 weeks
I had only gained 4 pounds and I was really proud of myself, then all of a sudden I was measured at gaining 10 pounds all together. I didn't lose any of that weight, but I was retaining about 3 pounds of water about two weeks ago that disappeared after I started watching my sodium intake.
DD 12/20/99, DS 12/14/12, M/C 9/2014, M/C 1/2015
PG weight gain is definitely NOT linear like the stupid books claim.
Last time I gained less than 2 pounds in the first tri, 16 pounds in 2nd tri, and then another 13 by 36 weeks. By the end it was coming on 2+ pounds a week. I only made it to 36 weeks so the total wasn't bad (30 lbs). But still, I swear I ate the same the whole way through!
I know other people that gained 10 pounds in the first tri and ended up with the same total.
This time I've gained 2x as much in the first tri and I'm hoping I don't follow the same pattern this time or I'm screwed!