are you saying something was swimming up and down your arm and you didn't notice it biting you continuously, or are you saying something was stinging you, but you didn't notice it at the time? Was your arm in contact with any rocks or stinging organisms? Personally, the bristles on the bristle worms cause me lots of problems if I leave them in my skin. I get that when I pick up a rock and touch a worm by accident or when I pull out a hand full of algae. The bristles stick into my skin, like the quills of a porcupine stick into the mouth of a wolf. The rash last for a couple of days, before my body breaks the hair like spikes down. Now, whenever I see a worm after handling something, I check my skin and pull/rub off all the bristles off my skin by using the algae scraper blade on myself, under running tap water.
Being under your arm, I'm assuming that was rubbing on the rim of your tank, maybe something sharp growing just under the rim?