i used to get them constantly as a child, don't worry, you will have one night of discomfort.
Sudocrem your bottom to ease the itching and take a piriton if you have one.
Boil wash all the bedding and towels along with your underwear and pyjamas. If you're worried, all the trousers you have worn for the last five days.
In the morning go to the Chemist and get Ovex, it's one tablet that kills them and another in a fortnight which kills any eggs that have hatched.
TREAT THE ENTIRE FAMILY.
Bleach the shit out of your toilet and sterilise the seat, they are contracted through toilets which is probably how you got them.
Wash your hands, constantly.
The grim bit is this - try not to scratch but if you do - wash your hands. The fuckers get under your nails and if they get into your mouth then the whole cycle starts again.
They're like headlice - uncomfortable but not dangerous. You're also not dirty, have you used a communal toilet?