What Causes Irregular And Heavy Menstrual Bleeding With Implant?

so im on the implant and have been with my boyfriend for 6years ... i dont have periods because of my implant. i started bleeding randomly out of the blue and it went on for about 2 to 3 weeks, i have no idea if i was pregnant and may have had a miscarriage but it is odd how i have been bleeding for so lon? can you help?