What Causes The Tailbone To Protrude Causing Slight Discomfort?

I am a fairly thin woman, weighing 110 pounds (5'5"). My husband always teases me and says that my pointy tailbone must mean that I am the "missing link" between ape and man! The other day I was talking to some of my girlfriends about it and I let them feel it. They were completely grossed out and said that I am growing a tail. It's not protruding by any means and has never caused me any problems except if I lie flat on my back on a hard surface for a period of time. I keep telling them that it's b/c I'm thinner and I just feel it more (they aren't as thin as I am). Can you tell me if this is completely abnormal? It's not actually pointy, it just doesn't tuck down in a rounded way like theirs! Thanks for any input.