My son is 13 days old and was born 4428g, He has been breastfeeding 3-4 hourly for approx 45 mins - 60 mins. I let him detach and then I burp him then I offer the other side. Sometimes he takes it, sometimes not. If he does I offer that same side next feed and if he doesn't I offer that side still for the next feed. He wakes up and demands a feed, sleeps very well and seems really content and has sufficient wet/soiled nappies.

Archer was born 4428, day 5 was 4185g (-243) then day 7 was 4200g (+ 15g) and day 12 was 4260 (+60g). So he has put on 75g in 5 days. The nurse was not happy with this and wanted me to express to see how much I got out.. I only got 20mls out of my left side which had a very damaged nipple but has since healed using a medela swing pump and was on it for about 15 mins, could this indicate I have low supply or something? He is very very sleepy at the breast but does suck for a good majority of it and I play with him.. He does not fuss or pull or carry on. Im really confused and feel like my body is failing me now as I thought we were doing ok

Any advice would be appreciated.
Amanda x