Blood in my cum

How normal is it to find some blood in your semen when you come occasionally? I'm mid 20's and the last couple of times I knocked one out I noticed there was a little bit of blood in there. I haven't dared masterbate since, I was hoping that if I left it for a few days it might sort itself out, as my penis has been sore and irritated the past few days as tends to happen once in a while.

I'm not sexually active so I doubt it's an STD. I googled it and found that it was fairly common in guys over 30, but there's no obvious cause. I have been under a lot of stress lately too, it's never happened before that I can remember.