Not gaining enough weight

Hi, I'm looking for a little advice. Or maybe just some reassurance...

My daughter was born at 3.92kg (8.6lb) and now, at 9 weeks, she's 4.9kg (10.8lb). We had a Well Baby check up earlier this week and the Child Health Nurse told us that she's dropped from the 60-somethingth percentile to the 30-somethingth. All of her other measurements are fine, and she was pleased with her alertness and how healthy and happy she looked. But she wants me to bring her to a feeding clinic next week to check her weight again and discuss our options if her gain hasn't picked up.

By options, she means supplementing.

I've been EBFing and had been really proud of us. I thought we were doing really well. She's had a good latch from the beginning, she feeds often and has lots of wet and dirty nappies.

This appointment really threw me, and I'm feeling like an absolute failure. My husband has been taking care of almost everything around the house so I can focus solely on taking care of our baby, and I feel like I've let her down. I don't know what I'm doing wrong, so I don't know how to fix it. I'm now stressing over each feed, worrying about how much she's eating and how often, which of course makes everything harder!

The nurse kept stressing that I'm not doing anything wrong... But I'm obviously not getting it right either.

Has anyone been in a similar situation? How did you resolve it? I'm now second-guessing every decision I make and feeling like a hopeless mother, and I need to get my act together pronto.

TIA