If you think you might be pregnant, go ahead and take a test. If the spotting continues, you might want to get seen in case the pregnancy is in trouble. If you take any type of chemical birth control (norplant, depo, or the pill), then some spotting is not terribly uncommon. Some women miss their periods altogether when they're on the shot. Keep an eye out for increased spotting, fever, or severe pain in your shoulders. Otherwise, I think you're probably fine.
If you are concerned about an unplanned pregnancy you probably have been having unprotected sex. if that is the case you should be concerned about STDs. If your periods are irregular in timing and flow consult your doctor. It could be any number of things, many easily treated.