I have an itchy anus after a shower, it can be quite intense then settles to a bearable itch, it mostly disapears after a few hours but will reapear straight after a shower, also i sometimes have a slight stomach ache and runny lumpy faeses?
Could be you're allergic to the soap you are using. I'm allergic to the ingredients SLS ( sodium lauryl sulfate) it's hard to find a soap or shampoo or even toothpaste without it but once I switched , no more itching or rashes. It's worth a try.