why do i have blood and blood clots in my urine after ejaculation?

im a 24 year old male.
last 2009, my partner and i were foreplaying. i've tried to insert my penis, but it was a struggle for me since hers was tight. at that time, we tried to do it standing so the pressure was on me. after the act,i have ejaculated and released clear fluid. minutes after, i felt the urge of urinating but to my surprise, blood came out together with my urine. there were blood clots which blocked the passage of urine so i always felt unsatisfied whenever i urinate. there were times when i really had the urge but couldnt get it out because something was blocking it. until i got to release the big clot that was blocking my urine by doing the valsalva maneuver (the act of pushing when pooping). i tried to let it heal and i didnt have sex nor masturbate for 2 weeks. after that, everything went back to normal.

last 2010, again, we were foreplaying. we dry humped but i cut the excitement because i felt like urinating. so i headed to the urinal and it happened again. blood clots were present in my urine and i didnt get to release everything. there is urgency but im not satisfied when i urinate. i got alarmed because i felt that my prostate was aching. it was really uncomfortable so i visited a urologist. he performed a dre and said that i have an enlarged prostate, due to the infection. he diagnosed me with UTI and gave me a prescription. i followed the regimen for a month and stopped the medication as instructed by the physician. im not convinced of having uti because aside from bleeding, i dont have any other symptoms of uti.

on the same year, it happened again while dry humping. (personally i dont think it came from dry humping. other people do that but they dont bleed afterwards.) i was really confused. so i visited the urologist again. this time, he made me take the medication for 6 months. the symptoms disappeared. it may have healed, though im still confused from where it came from and why im bleeding. my urologist diagnosed me with prostatitis. but then again, im not convinced with the diagnosis. and he couldnt explain clearly where the blood and the blood clots come from.

last week, it happened again. . i was getting excited and went with the flow of the moment. i dry humped my partner as foreplay. seriously, i didnt think all of this happened because of dry humping, so i repeated it. but then again, it always comes out after doing it. is it coincidence? now, im trying to figure out where it comes from? the prostate? how? why? another thing that i have noticed, once i remove the clot which blocks the urine, everything becomes fine. but when my penis gets hard due to cold weather or upon waking up, i notice that the clots and blood return when i urinate. why is this happening?
i have searched the net and havent found any article similar to my case. my urologist doesnt convince me with his diagnosis. please help me. i need an answer.