My period is late and I'm spotting; what's up with me?

This can be due to a number of different things including stress, weight gain/loss, thyroid, hormonal imbalance, infection, illness, medications, polyps, cysts, just to name a few. The most common cause is usually a hormonal imbalance. As you state you have been stressed lately and started up exercising, this can also be the cause. I would make an appointment with your doctor to have your hormonal levels checked to see if this is due to an imbalance.