What Does Light Bleeding With Negative Pregnancy Tests Mean?

im 22 years old. i was on the injection for 3 years, i came off last november and started to faintly bleed in february then during the next few months i was on for 7-10 days then off for about a week. i was last on at the end of august and have now been off for about 6 weeks, i have tryed 2 cheap HPT and both came back negative and have just tryed a clearblue digital and has also came back neg, what should i do??