What is it when a guy pees &the inside of the head of the penis hurts. He only has sex with one person and his girlfriend has been checked fordiseases?

Pain. You may have a urinary tract infection which causes inflammation of the urethra or urethritis.i would see a doctor to get a definite diagnosis and if necessary treatment.
More info. If you are not circumcised and have balanitis (inflammation at the head of the penis) that may be the cause. It would be a good idea to be examined. Good luck.