I think that most girth execises leave blood spots, since you are forcing more blood into the blood vessels than they can currently handle, these usually disappear within a day or two. These vessels gradually adapt to this increased internal force by getting bigger.
Warm up properly, build up slowly and make sure you stop jelquing, pumping, squeezing etc. immediately when a pain is felt to help avoid injury.
A certain amount of pain is felt because you need to get to the elastic limits of the tunica before girth will increase.
Thundersplace will answer all your fears if you spend some time looking through it.