What Causes Pain And Swelling On The Buttocks While Sitting?

A few days ago in school, I was sitting in one of those hard plastic chairs and found it uncomfortable (as I always do) so I grabbed another and propped up my feet on it. After about 30 mins I noticed by butt started to feel a little sore and stiff but for every other part of my body it felt comfortable so I just ignored the soreness. After about an hour and a half more spread out throughout the day of doing this I still felt sore and stiff but there was no pain and it disappeared almost as soon as I stood up. I got on the bus and went home with no problems big enough to remember. That was Friday, on Saturday I started to notice in my tailbone area it was sore and stiff (like before at school) but there was no pain until later in the day and it was very minimal; 1-3. On Sunday I started to notice it was getting worse and by the later part of the day was so bad I would have to adjust my sitting position every few seconds and started to take acetaminophen to help get through the day. There is almost no pain whatsoever when I stand or lay down but sitting for more then a minute felt like torture. It is now Monday for me and I have not taken any acetaminophen as of yet and I feel a lot less pain then yesterday (from 7-8 to 4-6) but it is still there. I felt around in my tailbone area after researching tailbone related injuries and found that on the left side of my cheek right beside the tailbone there is some swelling. I was wondering if you could explain what your best guess of this is and how long it will take for it to alleviate. Thanks for reading.