Perhaps you need to check its properly cleaned?You can use toilet wipes.Or after flushing I've heard of people dipping a wad of toilet paper in the clean bowl and use the wet paper to clean their backside.I think i prefer the toilet wipes personally.
Could be your choice of toilet paper. I can onlky use Andrex white, if I use any other or even Andrex coloured I have real problems, very sore,itchy and even bleeding. It was a friend who put me right on it, she told me about the different ingredients in coloured paper and the associated problems with it, hope this helps.