This happens very commonly when taking the morning after pill, or Plan B. It can and very often does cause a change in your period in which many women will get a period one week after taking the pill and then again when they are supposed to get it the following month. This is good news- it means you are not pregnant. Cramps can start early as a result of PMS, or it can be something completely not related to your period at all, like gas.
Remember to use protection next time to avoid the possibility of pregnancy! :)