What Causes Sore Swollen Labia Minora?

I am worried because i have a sore swollen labia minora, which is also itchy and throbbing. The area around it is also sore. There is no vaginal discharge. It is a problem that keeps reoccurring for a few months. It comes and goes. Have had tests at STD clinic which were clear. Its driving me crazy, losing sleep...seems worse than ever this time.