GREG E. COHEN, a podiatrist at Long Island College Hospital, hears the same story a lot: women complaining about a flaky red bump or a persistent itchy patch on a foot. By the time he sees them, they're embarrassed and horrified. A few years ago, Dr. Cohen, who also has a private practice in Brooklyn Heights, didn't know what to make of it, but these days he doesn't blink an eye. ''The first thing I ask is, 'Do you do yoga?' '' he said. As often as not, the answer is a resounding ''yes.'' In the last two years, Dr.