What Causes Inter Menstrual Bleeding When Nuva Ring Is Not Inserted?

I have been on nuvaring for over a year. In November I didn't get my period but I put a ring back in on the normal day I would have put it in if I had my period. After reading alot, I took ring out in case I was pregnant because I didn't want birth defects. Took a HPT- it was negative. I got my period about 9 days ago, had it for 3 days then stopped. Now 9 days later I am bleeding again? I didn't put ring back in...what's going on?