What Causes Irregular Menstruation Along With Brown Vaginal Discharge With Clots?

I have the nexplanon implant, for the last 6 months my periods have been very iregular but most recently I am on for 2-3 weeks, off for a week then back on for 2-3 weeks often with a clumpy brown dishhare. What could this be? I have had the implant for 2 years - not currently sexually active