Also, even something as stupid as stress can make your cycle go wack...so...it could be perfectly normal.
Sometimes, especially if just switching birth controls, or just starting a new one, it is normal for a woman to "spot" in between periods.
If you have been on the pill for a while, it could be an indication the hormones are not strong enough anymore.
OR if you are not on birth control, sometimes women "spot" after fertilization (AKA pregnancy). Not trying to scare you, it could be perfectly normal. But it wouldn't hurt to take a pregnancy test in a week or two.