What Causes Burning Sensation In Penis While Urinating?

I am type two diabetic. I take metaformin. I have recently noticed that when I urinate when have not drank enough water I get a burning sensation at the end of my penis when I finish. I have no back pain or any other pains. The burning never happens when I have been drinking a lot of water, only when water consumption has been diminished. Any help on this will be appreciated