What Causes Burning Sensation In The Vagina During Urination?

Hi I have recently started noticing the inner lips of my vagina had become very raw and today for the first time it burned when I peed. Could this be caused by too much rough masturbation. I've only had one sexual partner. My period is over 2 weeks late, I took 2 pregnancy tests from the drug store and one urinalysis from the health center at my university. All results came back negative.