Bleeding between periods has all sorts of causes. Some are quite serious, others are not serious at all.
- Where in your cycle are you? (Spotting sometimes happens around the time of ovulation.)
- Are you sexually active, and if you are, do you use any form of birth control? (This matters for a few reasons, like possible pregnancy and sexually transmitted diseases, to the birth control pill and other hormonal contraceptives sometimes causing spotting between periods. Vigorous sex can hurt the cervix can cause spotting as well.)
- Do you have any other symptoms? Pain, perhaps?
- Has this ever happened to you before?
Depending on your answers, I'd say either "it's never WRONG to see your doctor, even if the cause is likely benign" to "definitely see your doctor".