It can be a sign of pregnancy. Sometimes light bleeding or spotting occurs mid-cycle due to implantation of the egg into the lining of the uterus.
A young woman up to the age of 17 may have 3 months of missed periods simply because she hasn't reached her maturity level physically. Also stress can cause an "extra period" or missed periods. Going on Birth Control pills or patches can also change your cycle. It's best to see your doctor and let him/her decide what is going on.