This happened when I was younger quite regularly and continued right up to my mid twenties. There have been a few occasions since.

One, I have a name spelt the masculine way. It also doesn't help that my middle name is easily confused for being a male one too (one must look closely to realise there is an 'A' on the end). Two, I have always had short hair (it just won't grow dang it). Three, I was late to go through puberty. Four, until my mid twenties I was very athletic and consequently perhaps didn't appear the most feminine. Five my voice is alto and on the phone I am sometimes mistaken for being a man. When I was younger it was awful. Consequently I made sure to name my daughter a very feminine name, ensure she kept her hair long, and tried to find feminine clothes despite her own leaning towards sports and activities. I just didn't want her to go through the humiliation I did.