What Causes Burning Sensation In The Head Of The Penis While Urinating?

Hi, my name is brian im 25 years old.i had unprotected sex with my female friend once.the following day when i took a piss it was burning in the head of my penis..after two days it stoped burning and it was slightly itchy in the urethra no discharge bt if i force it to open its reddish inside with small dark veins n swollen a bit.i went to see a doctor,did the urine tests but it came out negative i got treated for chlamydia.he put me on antiuried creams but they make it worse.it has 5 months now.pliz help