Yeah, I think that's your tailbone!
My poor husband is 5'11" and maybe 145-150 lbs. His doctor keeps telling him to gain 20 lbs for the same reason. He knocks his tailbone around. He also can pop his hipbones just from laying in bed!
Tis true about the wooden chairs. But think about it this way, with a itty bitty bum, you can fit more pillows down in there!