[kermi]

Ok there are a couple of possible explanations here so to try and pin point what it may be, I have a few questions:

Have you given your nappies a strip wash?
Are they all fleece lined?
If not, do the natural lined nappies have the same effect?
Have you tried using Calendula ointment?

I have found that my youngest has a sensitivity to fleece inners on nappies.
To help with that (as we have a lot of fleece lined ones!), I strip wash them at regular intervals, use flushable liners and put him in natural lined nappies during the day.
Sometimes he still flairs up and I use a good amount of Calendula then but on the whole he is doing well.

Just a few thoughts there.