I have a sore tailbone, for no apparent reason. Googling around I found several mentions that having a "tipped" a.k.a. retrograde uterus can, in pregnancy, cause tailbone pain. Having found an illustration, I can see that possibly making sense; on the other hand, lots of things that appear to make sense in medicine turn out not to be true after all, so. Grain of salt.
I do have a tipped uterus, at least that's what They tell me when They have me in the stirrups. So I guess I can chalk it up to a Pregnancy Complaint. Maybe I need to find some funky-shaped cushion to sit on.