Elliott - my ds was an example of what mears desribes: a baby who was born "too big" (91st centile) and floated down the centiles until he was trundling along just underneath the bottom one! Bu then, both dh and I are small and (were!) skinny as children.
He took 7 weeks to regain birthweight and by 3 or 4 months was back to about about the 9th, where he stayed for a while before creeping up to the 25th - and then I don't know, as I stopped weighing him. At one stage I was even having to wake him to feed him (which was what the EBM was great for - it gave me the confidence that he WAS getting enough, so I did evetually stop the waking at night routine )
Throughout this he was manifestly happy, healthy and alert - he did have a few problems initially with latching, but even once that was sorted (and I was giving him LOADS of EBM), it still didn't change is rate of weight gain.
The miwdives who ran the b/f support group that I went to (actually AT the maternity hospital) did refer him on, just to cover themselves - but the consultant paediatrician took one look at this happy, healthy and alert child, told me not to worry and to stop the faff of expressing (I didn't, as it happened to be helpful, as i was going back to work when ds was 4 months old) and asked what my dad (an ex-colleague) was up to!
I know you've only recently had concenrs about his weight, but all I am trying (longwindedly!) to say is that babies follwo thier OWN growth charts. If you hadn't weighed him, would you have been worried? No. Would he have been referred then? Of course not!
Go along on Tuesday and allow yourself to be reassured by the doctor. You're NOT a bad mum - on the contrary, you are a fantastic mum.
FWIW, ds is now a happy, healthy 3.9 year old, who I THINK is about the 50th centile for both weight and height - I haven't checked recently. I'd like him to eat some more vegetables and fruit, but what mum wouldn't ?! He fills up on yoghurts, cornflakes, croissants, crackers and cheese - supplemented with the odd bag of crisps and chocolate buttons. I don't really care - he is obviously extremely happy, healthy and full of energy.