It is fairly common for women to have an "unusual" cycle now and then. I always recommend a pregnancy test as the first step. If that is negative, in your case, I would then wait and see what happens. If the spotting goes away and nothing else happens, I would not think much about it at all. If it becomes a pattern, and you have spotting with each cycle, or the bleeding is prolonged ( more that 8 days) or very heavy, it could be a sign of infection or other problem, and you should go in a see your MD.
I am assuming that you do not have a Mirena IUD as those only last for five years--if you have that one, you better go on in now!