What Are The Side Effects Of Using Emergency Contraceptive Pills?

My menstrual period is acting weird this month. Instead of getting my period, I have this light spotting and I can't recall a time when I've spotted before a period, except when on the pill, which I'm not. I was wondering if it may have been due to the fact that I've been riding my new bike very intensely lately up pretty big hills. Would this really affect my period in this way?