What could be causing a repetitive cold sore?

Several things: This is a problem that necessitates a face-to-face meeting with your doctor. This will allow him/her to examine you, ask specific questions. And possibly order tests to find out what's wrong and what to do to help you.

Many possibilities: Like: period, too much sun, getting run down etc. To prevent cold sores from occurring wash hands often. Maintain a healthful lifestyle (regular exercise, nutritious diet, adequate sleep, keeping stress at bay). Keep lips well moisturized & apply sun screen to the areas that your cold sores normally pop up. Talk with your physician about supplementing with lysine.