The reason you pull back on the plunger (or aspirate) is to see if any blood comes back into the needle, if it does you know it wasn't a great injection. I just repoke myself.

Sometimes you can actually see a little spot of blood there without aspirating. That means there's pressure from the blood going up the pin. Injecting in this case, is not a good idea. Just pull the pin out and try again.

Good luck