I have this same tail bone pain too. I asked my Ob about it last week. She said that all the pelvic floor muscles attach into your tailbone and for whatever reason it is causing pain. She couldn't tell me exactly why, but suggested it could have something to do with the fact that I still have morning sickness (at 18 weeks) and the vomiting may be straining those muscles. I'm not sure about that, but I guess it doesn't matter because there is nothing I can do to stop it.
The only thing she could suggest is putting a heat pack on the tail bone.
To me it doesn't feel like sciatica, for me it's not so much a nerve type feeling, more of a pulling feeling.
Sometimes it is so bad I kind of limp around, I feel like a grandma too!