also make sure you use a good sex lube when masturbating!! don't use shampoo, soap or body lotion as it can irritate your urogenital tract.
if you continue to have blood in your semen when you ejaculate or when you urinate, you will need to be seen for proper follow up.
Hi, it would appear that you have some type of injury rather than an infection from an STD as with them the symptoms would be a white or yellow puss discharge as well. Being rough with your penis could cause an injury like this. Since your very young it might be best to tell someone like your mother.