You wouldn't be thinking, you would know if his penis entered your anus.
as I told you in my first comment the risk was low.so don;t worry about it but get tested after 3 months.
i think the pennis was slowly going into my *** as the first stage goes when penis slips into the *** hole but it was only half or less way in and not fully he was into my ***.
did he enter his penis inside you or not?if not and you didn't feel anything and his penis just touched your hole from outside there is no risk.
if there was a penetration even for 2 or 3 sec there was a risk although low but a risk is a risk and you should get tested 3 months after your sex.
and it was slowly going into my *** but i felt no pain.