I Started Having Burning Sensation At The Tip (gland) Of

I started having burning sensation at the tip (gland) of my penis after an occasional sip of wine. My Doctors could not find out why after I have been through many test for STD or enlarged prostate came all negative. They gave up on me not knowing the underlining problem. It is painful and bothersome for about 5 minutes or so and settles down if I have a large glass of water. What is the issue here?