I Have A Large Round Swollen Bump On The Back

I have a large round swollen bump on the back of my heel. very sore to the touch. I have had it for about 2 months. also very tender when walking. I walk 10-15 miles on cement all day at work. I'm a female in my fifties and physically active and good health except for severe pain in back of my heel. do you have any ideas on what this is?