Dropped from 75th percentile to 50th percentile

This is baby’s weight only. Baby is one month old and her length and head are still on the 75th percentile. Her doctor doesn’t seem concerned but I am. She was expected to gain 18 ounces but only gained 14 or 15. Does anyone have experience with this?

My first baby was formula fed and gained a lot (more than he needed to) so this is very new for me. Baby’s diaper output is good and she seems happy after eating except in the evenings when she wants to nurse more but I am told this is normal cluster feeding before she takes a longer sleep and not indication of her not getting enough milk. I am looking into more breastfeeding support through WIC because I am really distraught over having to maybe give her formula soon. I stopped breastfeeding after a week with my first out of fear I couldn’t do it/feed him enough and I don’t want to that again.