bruising is caused by blood vessels breaking underneath the skin. while nematocysts are stinging cells that enter your skin and release something into the surrounding flesh and generally irritate the skin causing bumps, rash, swelling, itching, redness, etc - it is possible that something stung you (which, by the way, you may not have felt at the time of the sting) that in addition to other trauma, caused blood vessels to break.
A picture would be worth a lot, though. I have been stung a few times (once by an anemone and once by jellyfish) and there was a distinct pattern to the trauma. Do you see any pattern to the bruise? Is there irritation in addition to the bruise?