magnesium story... yeah, sure... maybe if you hadn't had any magnesium in the last 4 years, then that COULD be the source of it...
Asprin is a potential culprit, if the darkened region of the bruise is spreading, but not the "pain region." However, that, in itself, isn't going to spread it too much.
It all comes down to where the bruise is located. Its likely that you hit the bicep/tricep circulation branch, which could explain the high degree of internal blood loss, but someone would have to look at it to be sure...(i can't see your bruise, cant tell ya) Just give your lymphatic system some time to recover the lost blood, it'll go away... Might be an ugly yellow-green color first, though