Assuming you don't have threadworms, you most likely have excess acid which
causes the same feeling.

You can get Losec (a proton pump inhibitor so it says) for excess acid. Been
on it since 1990 and it is marvellous. If you have excess acid and don't do
anything about it, like me you will eventually end up having that itch feel
like you are walking around with broken glass in your arse every time you
crap because of the acid no matter WHAT you eat (though eating dry white
bread helps it somewhat). Get the acid checked. It not only can be a sign of
something insignificant, it can also be a sign of cancer. Better to know
than to wonder about it to death.