daily two times..any affect?

I have been having constant irritation at the head of my penis right at the urethral opening, for about a week now. I went to a urologist and did a urine test but everything came out to be normal. The doctor prescribed me some antibiotics and neurobion forte. Its been few days now but the problem is still there. The burn is always there and my mind is going nuts over it. Visually it can be seen that a slight decoloration is there right at the opening of the urethra. Its like the black eye you get after a punch but i don't recall any injuries. I havent had sex and I dont masterbate frequently. Please help.