Weight gain

Hi, my baby girl turns 7 months old tomorrow and we just recently went to her 6 month check up. She weighed 13 lbs and the dr was concerned and wants to see her back in a month. Does anyone else have a small baby that could put my mind at ease? She’s breast fed most of the time unless I’m at work and she maybe drinks 3 oz out of bottles (she’s never been a fan of bottles) and we’re also feeding her solids 3 times a day.. I don’t feel like anythings wrong since she’s meeting all of her milestones and is happy most of the time unless she’s tired or going through a leap. Also at birth she weighed 6lbs 10oz