Gross hematuria, end of urination. blood samples, urine samples done. no infection. "tense" bladder, lower back pain, occasional scrotum/testicle pain?

Hmm...: I bet you've got a little vascular malformation that's grown in your prostatic urethra. Time to get with a urologist. You're a young man, probably in the peak of good health, and pain around there is surely a nuisance. We can figure it out and fix it, I'll bet. Best wishes.

Terminal hematuria: Could be posterior urethritis or prostatitis, assuming urine is clear at first. Advise consult urologist. Will likely order kidney & bladder ultrasound + possible cystoscopic examination. Cystoscope is a long thin instrument for examining urethra & bladder.