Coccydynia is not fun.
If that's what it is, stay away from the "donut" cushions. Get one of these instead:
Amazon.com: Coccyx Cushion - Extra Soft: Health & Personal Care
In fact, you'll need several - one for the car, one for every office chair, etc.
That's
if you have coccydynia. You absolutely need to get checked out.