Nuva Ring

Am 24 years old and have been on the nuva ring for almost a year now. I have been having problems with my period for the past few months. The last few months it has been very light and only lasting two days, last month it was very heavy and lasted four days. This month I got my period on the very last before my ring should have gone back in and it was very light, spotted and brown. I have been taking a pregnant test at least twice a month, and nothing.  My yearly check up is next week. What could it be?