Hi the skin between the vagina and anus burns when my urine touches it. only burns every so often. what does this mean?

Inflammation: You are getting inflammation in the area. If it happens after intercourse, there may be some abrasions. You might need to use a water-based lubricant. If it doesn't just happen after intercourse, you may have an external yeast infection. You might try some over-the-counter antifungal cream. If this does not help, you may need to go get checked by your gynecologist.