leg cramps while exercising

I have been walking 4-6 miles, 5 days a week for the last 3 years and every once in a while I get cramps in my shin and ankles that unless I stop every block or so do not let up. I warm up, I have good shoes, I do everything that people suggest and it still happens. I find that it is worse when I ovulate and when I'm on my period. I eat a healthy diet and drink lots of water (with a once in a while splurge) I have a small bunion on the leg that cramps worse and wonder if that has something to do with it. I sometimes have to sit down,take my shoes off and bend my toes down to get any relief. Its frustrating and would love some insight, stretches, etc...