What Causes Penis Pain?

My symptoms started 4 days ago. My penus hurts a lot except when I pee. When I pee, everything seems normal and then a couple seconds later, it starts to hurt alot. I have no noticeable legions (checked by 3 doctors). My doctor mentioned it could be an STD inside the uretha which is why no legions are visible but wouldn't it hurt considerably when I pee? Is there any STD that doesn't hurt when you pee?