Spotting instead of normal periods is a very common phenomenon amongst females and generally the cause is not serious however one must look into the cause for it.
The two most important reasons are
1... Use of birth control pills.The hormones present in these pills result in alteration of hormone balance of body.
2.. If one is actually pregnant.This suspicion is more if one is sexually active.
However there are other reasons like vaginal or cervical infection
,cervical polyps,any disease altering the hormone balance of body,use of blood thinning medicines.
You may rule out certain causes from this list and in case still unable to zero down the cause,consult a Gynecologist for proper evaluation.