Help! The Doctors Can't find what is wrong with me!?

Ok, so. I asked a question a little while back, asking why the heck it hurts when I pee and a little while later, after I was done peeing, some blood would... "trickle" out. It scared the crap out of me, and so I finally got the balls to tell my parents. I went to the Doctors and did 2 "UTI tests" and one kidney test. (ultrasound) BOTH CAME BACK NEGATIVE! The doctors don't know what is wrong with me, and I am very scared. Please help me. Tell me what is wrong with me. Oh ya, and I am 16 years old, male.