Because of exessive masturbation, after urinating i don't feel as if the bladder is empty, but no more pee comes out either. why?

Likely Prostatitis: Repeated ejaculation can incite swelling in the prostate. This results in the bladder feeling as if there is still urine present even after finishing. The swelling can also result in decreased force of the stream, urgency and sensation of incomplete emptying the best way to resolve this is to abstain for several days and see if it goes away. Aleve/ibuprofen or Aspirin and increased water helps.