I don't understand what you mean by "if there's blood in my dose".
If you're talking about IV, you get blood in your dose as part of registering. It's normal and since it's your blood and not coagulated it's fine.
Anyway, you said "sometimes", how often does it happen? It might be there's a particular cut your body doesn't like that you are sometimes exposed to but not others. But that only makes sense if you have no problems with some batches. If it's just random with every shot that's not it.
Beyond that I don't currently have a theory.