Slow Weight Gain. Should I worry?

My daughter has been having difficulty gaining weight ever since we've been going to her ped. Her birth weight was 6.9 and 3 months later, she's only 9.8. There was a time we went to the ped and she had only gained 4 oz in a month. We had to supplement at the beginning because she wouldn't latch, but now that she latches, we're strictly doing breastmilk. I went to see a LC today to see how much she was getting from my boobs and the LC said the amount was good, but she only gained 4 oz again since our last ped visit, which was on the 3rd. She's exceeding all of her milestones so i'm thinking maybe she's just a slow gainer, but should I start worrying or discuss supplementing with formula again?