Both my LO's were below the 5th percentile at birth (4.5 and 5.1lbs). They have grown wonderfully and are both now in the 60th percentile (adjusted age, they''d be 75th+). DD has not gained anything in the last month (we have a home scale and I weigh them each month on their birthdate). This is despite having a huge appetite increase, taking all her bottles, and eating 3 solid meals. DS has actually lost several ounces. Granted, he's been battling late-onset reflux and was vomiting 1-2x a day for a couple weeks. But, that has stopped with reflux meds and he hasn't thrown up in almost 2 weeks. His appetite is much improved also.
I don't know if it's just a fluke? Or if it's because they are becoming so active at this age? They've just been such steady growers since birth, so I'm trying not to worry and read too much into it. I was just stumped when I put them on the scale, despite how much they've been eating lately.

Re: Anyone's LO lose/stop gaining weight around 9-10 months?
Luke had the same issue at 10 months exactly. He still hasn't gained a pound or grown more than 1 inch since then. He was tiny to begin with (5lbs 18in) at birth and his dr isn't worried.
He lost 2 pounds in a matter of weeks last month and we were watching that, but he gained them back once he was feeling better and hasn't lost/gained any since then.