If you want to send me a photo I can pm you my email addy to see if it is anything like Ben has had.
My top tips at clearing the rash and keeping it at bay is
1) Don't use wipes unless for a poo, wet nappies just off and dry nappy on
2) Use Boots nappy liners (wash the wet, flush/bin poo ones)
3) Try using disp wipes - I know they are horrible, but I haven't gone back to washables as they are still working for us and only used even now for poos
4) Wash at 60 degrees never lower using bio powder.
5) Easier said than done, find the right combination of creams for you. For about 8 months we swore by applying Dakatcort (for fungal nappy rash) then a thick coating of Bepanthen and that cleared up really horrible rash all over and including bleeding blisters. Recently I have discovered epaderm and use that as a barrier. The creams were literally trial and error and I bought everything mentioned on here when I was at my wits end and going to give up on cloth but in tears at thinking that was an option. We lived at the gp's and ended up getting all creams on prescription but it took ages to get the right quantities - each week we would use a whole tube of bepanthen (large £7ish one and 1 tube of daktacort). Benjamin also had swabs taken and sent for testing incase it was anything worse than normal fungal rash and as a toddler we had a dematologist appointment at hospital as the gp had no idea what it could be and that was the epaderm cream which cleared that up.
If you are solely ff at the moment, at least that is one avenue ok as what's going in / being digested hasn't changed.
I also found it was easier to stick with the same type of nappies and system so if you are switching types of liners keep that the same for 3 days min to compare if getting better / worse