Depending on your age, it could be a number of things. If you are over 35, you could be in the very early stages of Peri-menopause. You could also have Endometriosis or other, similar issues (I can't recall all the names off the top of my head). Also stress can cause changes to and in your period. Any way you put it, your best bet would be to talk to you OB/GYN or your primary doctor about it. They can assist you in finding out what the problem is and work with you to fix it, if it is fixable.