Very probably your bleeding is your body adjusting to the Nuva Ring. When you switch formulations of HBC, just like when you start for the first time, there can be an adjustment period of a few cycles. Additionally, my guess is that Nuva Ring has a lower progestin potency** than does your last week of OTC, so it may have your body going, "Okay, hormone drop. Time to bleed now! Wait? There are still hormones? WTF, people? What is going on and why did I not get the memo?"

** It's hard to compare directly since they're different progestins entirely, but Nuva Ring is generally marketed to be a very low dose of hormones. And given that a lower dose version of OTC exists (OTC-lo), it makes sense that the regular OTC is not so low a dose.

It's also difficult to predict when your bleeding will stop. It may taper off or go away completely within the next few days, or it may hang around in some form until you next take a hormone-free week. But overall, it's probably not an indicator of how your body will react to stacking -- from Nuva Ring to Nuva Ring, which would be a consistent level of hormones throughout -- by the time your body has adjusted. Stacking may work out for you or it may not, but this bleeding is not necessarily a sign of All Stacking Bleeding Yet to Come. ;)