I can't remember if I did with Charlie (never had the chacne to sit down for long enough!) but I defiantely did with Olivia. My job at the time involved a lot of active work on construction sites in the morning, then a lot of sitting at the laptop in the office all afternoon, and through the third tri it was horrific! The pain in my tail-bone was so bad that I felt like I could barely stand up again! Swimming after work helped it all "unkink".

I think it is just the body trying to adjust to the new levels of weight distribution!

I am pretty sure it is totally normal, but check with your OB/midwife/Dr at your next appt? Just in case?