2 yo with runny poos and a very sore bum - breastfeeding related?

[Goldrill]

DD has been having quite runny and frequent poos for about 10 days now - no other problems - eating and sleeping as well as usual. It is causing her to have bad nappy rash, however, which she has never had before, to the point where she started screaming before we even got her in the bath tonight as it obviously hurt so much, poor love.

Only obvious thing which is different to usual is that she has started breastfeeding quite a lot more since DD2 arrived 7 weeks ago - and my milk will be different as it's for the newborn. Am wondering if this could be causing the problem?

Any advice/ideas gratefully received.

not sure of cause for her, but my DD has the same at the mo due to teething/antibiotics. her bottom looks like she sat on a hedgehog - best cure is vaseline and more vaseline, seriously, just keep applying it every time you clean her!

forget about the bath, it would just make it worse, clean her body with a wet flannel and for pooey bottom wipe off what you can again with some very soft wet cloth, or loosen dried on bits first with vaseline. when her bottom is clean try leaving nappy off for a short while

if it was the breast milk wouldn't baby have the same problems?
I don't know, maybe it does effect DD1, but there are so many bugs going round, I think it's more likely she's got something lurking, even if she's ok for now.