Physical or emotional trauma can disrupt the
menstrual cycle. So if it had not been for the accident, your periods may not have stopped for that long. Also, in the early post-menopausal years (5 or so), hormones can still fluctuate quite a bit and even cause bleeding that is oftentimes nothing serious.
Another thought - some medications can disrupt the menstrual cycle. So if you were on meds due to the accident, that could have thrown everything out of whack.