If you're not at the right age for menopause and your periods haven't been coming for a year or two, could it be that your body-mass index is way down and you don't have enough fat? If your body has been really lean (not as much fat as the average person), periods can stop. This is because your body thinks it is starving, and doesn't want you to have a baby in such circumstances. But then if a woman puts on a little bit of body fat, the periods can come back again. (This can also happen to a woman who has been under stress that feels life-threatening to the body, it doesn't necessarily have to do with only fat levels. If you have been under a lot of stress and it has abated, your periods can come back.) Since you mentioned starting to work out, it occurred to me that possibly you have been really lean and began to work out due to putting on a little weight. If that is it, look up the consequences of being so lean that your periods don't come -- osteoporosis is a big problem.
You haven't had a period for a year or two, because ... ?
If the reason is menopause, then it's not too surprising that you might have some random light bleeding, maybe even because your workouts have rejuvenated your hormones just a tiny bit and you had an ovulation. Probably more commonly, women who are menopausal do just sometimes have an unexpected period or light bleeding every now and then, along the path to being completely done with periods.
If you are not at the right age for menopause, have you seen a doctor to figure out why your periods are AWOL?