What Causes Vaginal Ulcers While Using NuvaRing?

I HAVE BEEN ON THE NUEVA RING FOR ALMOST TWO MONTHS ABOUT TWO DAYS AFTER STARTING I GOT AN UCLER ON THE INSIDE OF VANGINAIL WALL AND HAVE GONE TO DOCTOR AND BEEN GIVEN CIPROFLOXACIN AND WAS ALSO TESTED FOR ANY STD ( THE TEST WERE ALL NEG) THE SECOND MONTH I INSERTED THE RING ONCE AGAIN TWO DAYS LATTER I GOT TWO MORE ULCERS . MY DOCTOR SAYS THAT IT IS NOT FROM THE NUEVA RING BUT . IM SATARTING TO THINK IT IS .SINCE HE DOESNT HAVE AN EXPLAINATION FOR THE ULCERS . ANY IDEA WHAT COULD BE CAUSING THESE ULCER ?