Are you having any kind of discharge besides the spotting? Sometimes the discharge tells us something.
I don't know how old you are but if you are over thirty, it could be fibroids causing the spotting. They are totally normal.
I personally have a condition called uterine hyperplasia and displayed the same symptoms you have but this condition is uncommon and very rare in women under the age of forty.
Basically its the walls of the uterus are thickening due to hormones gone haywire. I am getting a biopsy in may (this condition can lead to uterine cancer if not treated) and then being put on meds if its early enough not to require surgery.
Here are the women most prone to uterine hyperplasia that I have read about.
Women who are
white,
over age 40
have not had more than one pregnancy
are overweight (the more overweight the higher the risk)
are diabetic
an early warning sign besides the light spotting is watery discharge and later a yellow pus looking discharge.
Anyhow I think i'm trying to say there are different reasons that can be causing the spotting between your cycles. Give us more information about you and I am sure someone on here will be more helpful.