DD was exactly the same for the about two weeks after she first started wearing pants. I wondered if it was an allergy to the elastic around the edge of her pants - I was trying to only use our ABK ones as they have a stretchy fabric edge rather than exposed elastic, but MIL bought a load of cheap pants from Matalan and DD always seemed to be wearing them when I picked her up
I asked her to try and stick with the ABK knickers as much as possible, and I also used Sudacrem or Waitrose bum butter. DD had never had a rash in nappies, but this was horrible, like blisters which the scabbed over - it was so sore and she hated me touching it to clean her.
It seems to have pretty much gone away now, she still has it occasionally, but not as bad as those first few days - I have some more ABK pants on the way from the latest co-op, and I have also bought the pattern to make my own pants for DD as I'd like to have enough for her to be full-time in those - problem is that somedays we go through 3 pairs of pants