Even for a person with a regular cycle, stress - emotional, physical or nutritional - can change the timing.  For example, someone who is extremely athletic or not eating properly can prevent your period from coming altogether.

If you are spotting, chances are you are getting your period early, and the spotting signifies the end of the Luteal Hormone phase. Back pain and stomach cramping are common PMS symptoms for many women.  I wouldn't worry about these symptoms just yet, see if you period comes within the next day or two.

If you want to better understand your natural cycle, I recommend the book by Toni Weschler, Taking Charge of Your Fertility.