My midwife told me that an adjustment from a chiropractor can help, but depending on how loose your sacroiliac joint is, it could go back within a day. I had the same pain for a few weeks. It usually wasn't there in the morning, but if I did anything even remotely strenuous (walking up a hill, vacuuming, etc) I would be sore the rest of the day. By morning it was fine again. It got to the point where I couldn't do anything without aggravating it. Luckily the pain has gone away for the most part.
Your body is getting ready. Your tail bone is in the way of were your little one has to come out and some women complain of pain or shifting before hand. My chiropractic has told me about it since I have issues with my lower back and pelvic bones due to breaking it. You might look into getting adjusted it could help.
Oh it's because the bones or joints are getting ready for the big stretch. It's sore
I used to suffer from that tailbone pain and sitting posture usually helped me, sitting straight not slouched. Also not sitting for too long.
Now recently I heard a snapping sound during the night and woke up with what I could swear is a cracked pubis bone. Bloody sore and turning in bed is a nightmare. I don't even want to mention getting out of bed eish.