I still worry

Does anyone else still worry about LO even though the pediatrician says they are fine. My son was born 5lbs 14oz. I breastfed for the first month and that was way too long. I had plenty of supply (pump and bottle after 2 weeks to measure) but by 1 month my son had only gained 6 oz. We switched to formula for a week so the dr would know exactly what the baby was getting and over those 4 days he gained weight perfectly. So we switched (still haven't stopped blaming myself). He was already small at birth but now he isn't even on the growth chart. 3% height, 3% head and <1% weight. He seems to be developing well but I can't help but worry (and I feel terrible for taking so long to switch)