I have a simular problem. Your symptom is most likely caused by infection in your Prostate. This is also know as Prostatis. It can be either acute or chronic. See your GP or Urologist to get it checked out.
Hope this helps!
I would. not sure but if it keeps up i wouldn't wait around. look up on the internet about it maybe go to ask jeeves and type in blood in semen it may be able to help to. good luck