It's hard to say. It could be a lot of things. You don't say how old you are but if you are in your 40s, it could be that you are now pre-menopausal. Also, regardless of your age, it could be hormone related. And, yes, stress can affect your cycles. You don't say if your periods have also become heavier, if that's the case it could be fibroids. Fibroids can cause you to have shorter and heavier cycles.