Hi durianfan, just wondering, are you offering baby milk by bottle? Usually that can cause nipple confusion...it's best to avoid bottles till a few months later but in the meantime if you are feeding EBM you can feed using a syringe.
Other than that it could simply be baby is learning how to breastfeed as breastfeeding is a skill that both mum and baby need to learn and it takes some time before both get the hang of it. Sometimes if mummy's milk flow is too strong and fast (especially when letdown occurs), baby hasn't quite learnt to coordinate sucking and swallowing when a downpour flows into their mouth... When this happens you can take baby off boob and wait till letdown is finished before trying to latch again.
You can find some great tips here:
kellymom.com :: Help -- My Baby Won't Nurse!
And if possible, please see a lactation consultant.
I hope it gets better for both of you