It sounds like pinworms. Pinworms come out at night from the anus (usually between 11pm and 1am) and lay their eggs on the buttocks. Just been through this one with my daughter, so know all about it. They are small, threadlike worms that can often be mistaken for lint or pieces of toilet paper.
To get rid of them you have some options. You can do a regular parasite cleanse and/or you can take Pin-X or Reese's Pinworm medicine (over-the-counter drugs).
Also of note, if one family member has it, chances are that they all do so you need to treat anyone in the household. I used Reese's for the whole family (one time dose) and started a complete parasite cleanse for myself. I also just ordered humaworm for the whole family to get any strays and other types of parasites.
Another thing to note is that you need to do some heavy cleaning of your living environment in order to get rid of as many microscopic eggs as possible. Don't sweep! Wet mop the floors, vacuum the rugs and floors, wash all the bedding, underwear, and bed clothes in hot water if at all possible and dry on high in your dryer. Pinworm eggs can live up to 3 weeks without a host. Also, when you do the parasite cleanse and/or Pin-x it takes about a week to kill them. So, you need to continue to cleanse your house (changing bedsheets at least every other night, damp mopping the floors, and remember to wash your hands often). Another helpful thing is to cover the buttocks and all surrounding areas in vaseline before bed. The females get stuck in the vaseline and die and cannot release their eggs. I did this with my children each night for a week.
For itching until they die, you can take an Epsom Salt bath at night which helps.
Also, I used a paper towel and wetted it with collodial silver and put it in my panties - kills and stops the itching till their gone.