What can it mean if u have had light spotty periods for 3 months that have small dark blood clots at times? periods have usually been regular

Could be normal: Maybe some hormonal imbalance. At your age cancer is uncommon but never the less you should not be having this problem for 3 months. Please consult with a gynecologist.

Pregnancy ?: If you have been having regular periods up to this point and now there has been a significant change, pregnancy or misconception is a possibility. If you are sexually active then take a test. If not, i would recommend seeing you gynecologist for a routine exam and consultation.