Go see a doctor. Blood coming from anything other than a cut (or menstruation in girls, which isn't entirely blood), isn't generally a good sign.
And if it's coming out in your urine or interject then it can be quite serious. It can be a physical8cal injury or caused by infections or inflammation. It can also be linked to cancers of the p-spot, bladder and testes.
It might just go away, but it's best to get it checked, for peace of mind and help, particularly if you have other symptoms, like pain when ejaculating or urinating, or blood in your urine, or if your snake or urethra are painful.
http://www.nhs.uk/Conditions/blood-in-semen/Pages/Introduction.aspx