Was at the chiropractor today. We also get physical therapy and massage. I was about 5 feet from this older lady and she thought I was a boy. In just a tee shirt (funny enough it was a girl scout one) Even with my high voice. I was laying on the table talking about how the divet above my butt hurt and she was like "boys don't have those only girls do".

Congrats on being read as male. It's extra satisfying, and funny, when the clothing isn't even something masculine, huh?
Out of curiousity, Googled around to find what the term for this might be. If the divot in question is a "sacral dimple", she's wrong (even setting aside matters of sex =/= gender) - cis males get them too.

We have two of them like this http://www.beautynstyle.net/wp-content/uploads/2013/08/back-dimple-tattoo-9.jpg

Those aren't unique to women. Lots of men have them.