Felling Hard On Butt. Unable To Walk. Neck And Arms Are Really Sore. What To Do?

I fell hard on my butt at school in the gym. It was raining outside, and so the ground was slippery from people's shoes. My feet came out from under me, and my rear end, back, and neck slammed into the ground (in that order). I got up immediately and was able to walk to the bleachers for the assembly we were having, but had to sit in the very first row of bleachers because my tailbone area was in severe pain. I could barely even walk. It kept hurting for the hour long assembly, and so I went to the nurses office straight afterwards. I had to be picked up by my dad because by that point I couldn't stand, sit, or walk without extreme pain. For the rest of that day I felt pain in my tailbone area and when I took too big of a step, I would feel like my tailbone area would just seize up and I felt like I was being stabbed. The next day was Saturday and I found that I could barely even go pee because every time I pushed down to pee, I would seize up in my back again. It's Sunday right now, and I'm afraid to go poop because of all the pain I know it will cause me. My tailbone is a lot better than it was on Friday, but it hurts to sit for long periods of time and standing up is excruciating. Also, my neck and arms are really sore, almost like whiplash. I can barely move them. What should I do? Why am I this hurt from just falling over? I'm a teenager, if that matters.