What Causes Brown Vaginal Bleeding While On The NuvaRing?

I took my Nuvaring out on the 1st of January and began my period on the morning of the 4th. I reinserted the ring on the 8th and finished spotting on the 9th then two weeks later on the 22nd I began light brown bleeding that is still occurring now on the 26th. I have been on the ring for 2 and a half years and nothing has ever happened like this before and I have been sexually active with my longtime partner the whole time. What could the bleeding be from?