Did my tooth rot behind a bracket?

It is possible that a cavity can form behind or around a bracket. Hopefully this would have been noticed by your orthodontist, dentist or hygienist before it reached the nerve but its still possible. This can happen if the bracket has de-bonded but still attached to the tooth. In this case bacteria can get beneath the bracket and becomes impossible to clean off resulting in a cavity. There are other issues that could cause this area to be sore so having it checked out by a professional would help determine if this was the cause. 

While the possiblity exists to get decay underneath brackets during orthodontic care, it isn't common.  It is much more likely AROUND a bracket, but still uncommon.  Hygiene is extremely important, so is your monthly visit to the orthodontist.  During that visit, they should notice decay, but could still miss lesions.  See your orthodontist ASAP.