if there are problems with bf, including slow weight gain, painful nipples, baby feeds really frequently but falls asleep at breast and you are literally feeding every 15 mins day/night (not normal!!!), incorrect latch, dummying but not suckling - they may have TOUNGE-TIE!!! 5 out of our 6 had problems with bf - 3 were diagnosed and I suspect 2 possibly had it.
get baby checked for tounge-tie and if diagnosed she/he can have a little op to sort it.
btw tounge tie can effect bottle fed babies too - my DD struggled with bf as described above, but bottle feeding was even worse until this was sorted!
DS4 was ELSC and had jaundice at 5 days, had to give him formula in the hospital as he was too sleepy bf.
carried on with bf plus small amounts formula 1 or 2 x a day till he got better, then went back to bf only.
at 2.5 weeks and again at 6 weeks I had mastitis, so I went back to mixed feeding to help recovery and to keep my sanity!
I ended up stopping bf him completely at 3 months as I was still recovering from C/S, had 3 other kids under 6 and 3 school runs/day - 1 in infant school, 2 in nursery and it was all a bit much!
btw you can feed through mastitis, best to have some painkillers and use Avent nipple shields!