
I have on some occasions (currently also) found that the birth fluids have been so caustic as to cause sores on the backs of the thighs nearest the vulva. This is where the coat became soaked during whelping and the hair has come away and left the area sore.
I have used Vaseline or baby oil on the area after cleansing carefully. Keep checking the trousers for any gunk from the post whelping discharge and keep the area clean and combed, and cut away any straggly bits of fathering that gets mucky.