Weight gain: when do I worry?

This is my first pregnancy (via ivf), today I got week 19 with our baby boy.. exciting until I weighed myself!! I started the pregnancy at 146 lbs at 4 weeks ( I am 5’6 and in the healthy/normal BMI). My OB advised 25-35 lbs of weight gain total, and my little app (that maybe I shouldn’t listen to, but alas), tells me at this point I should’ve gained 10-15 lbs. Well, I’m a bit over and have gained 20 lbs. I don’t feel it, and I don’t necessarily think I look it. I maybe have added some fullness in my arms and thighs but the rest of me just feels like all boobs and belly. I know… 20 lbs is NOT concerning. Here is my real concern, I feel like I put it on in big swings? Ex: I’m up 6 lbs this week? I’ve had other weeks where I’ve gained 4 lbs.. I guess I’m a little confused! I did check a few days in a row for fluctuation and it’s been consistent. Is that normal?? I’ll FU with my OB but asking for personal experiences. This is my first rodeo - really just trying to gauge how much this whole weight thing is really important