(Warning, I'm going to describe this in detail, so it may be a bit disgusting)
I've had this itchy bum for ages, like years, it doesn't affect me at all except for the fact it itches like crazy. I am female, 20s, never had a partner before, so STIs etc. are out of the question. I did a faecal sample for worms once and it came back negative. I did notice there were white spots in the faeces when I collected the sample, but it was not the "specks" people seem to describe for worms (and no movement either), it was like the shape of a corn kernel but a creamy white colour. I know for sure I didn't eat anything that could produce something like that, and I did notice those white spots a few other times in the faeces. I did put those spots in the faecal sample too just in case (2 large ones, they were very obvious), so either the people who tested the sample were blind, or they are definitely not worms. I also poked them a bit (not with my hands) and some whitish goo came out. I don't have any other symptoms, such as changes to appetite or pain, and the itch is not restricted to night time. It seems worst if I lie on my stomach, pretty bad if I lie on my side and not there at all if I am standing, sitting or lying on my back. I don't know if this is related, but I also get some vaginal itch sometimes, but I feel that is mostly caused by pubic hair in weird spots. From the stuff on the net, I'm thinking the itch may be a fungal infection? But I'm super healthy otherwise and have never been immunocompromised so I'm not sure what the chances are.
Anyone have any ideas?