I broke my coccyx (and many other bones) in a motorcycle accident. I could feel that it was broken and the pain was excruciating.

The best advice I can give if you are broke is to head to Carolyn Downs and get some medical help.

Your coccyx may not be broken, however. The glutes are big muscles and when they get bruised they can hurt like hell for weeks. The getting worse after two days part is very common with bad muscle contusions.

Has your ass begun to discolor? Reds, purples, yellows, etc?

If you must sit down, like at work, use a donut like sublevelthree mentioned. Whenever you have the chance, lie down on your belly or on your right side. Take some mild painkillers. Use some ice. Take it easy. That's probably the most you can do.

I had a similar accident a while back, although it mostly jarred my back and didn't cause any swelling. I pretty much stayed in bed for a couple of days until it felt better.

If you've broken your tailbone all they can do is give you a donut to sit on. There is no " butt cast"
If it's still hurting by all means see a dr but ask them before they take the xrays ( if you have no insurance) they may just tell you to get the donut and forgo the expensive X-rays

If it keeps bothering you see a doctor.

Freikja, srlsy, you should know that q-land is not a safe place for responsible medical advice. Do you really have a broken coccyx? Then go the hell to an emergency room right now!!!
Or do you not have insurance and you don't want to pay $500 to get some x-rays and be told that "yup that must really hurt, here's a prescription for tylenol-3"?
These are pretty much the only two options (other than giving them a fake name at the emergency room) and this is the worst part of being uninsured, do I go or can I ride it out?
Your call, if it was me and the "other afternoon" was less than a week ago and I could still walk (if you really have a "broken ass" you're not walking anywhere) my uninsured ass would ride it out (and walk it off, hah!) arnica, weed, booze, heat, ice, rest, and movement are all non-prescription and awesome for that sort of thing.

Only you know though, if it hurts that much go the fuck to a doctor, get a butt cast, and sit in a wheel chair for six weeks. Odds are though, if you're asking the internet instead of crying face down on an examination bench you don't need to.
It's weird how it hurts more two days later but
you know how when you go running for the first time in awhile and the second or even third day is so much worse? Lots of times bodily trauma is like that.

I really hope you feel better soon, but like I said, your call, and q-land is not a doctor.