I had the same issue with my 7 plus. Got really hot at the top backside by the camera. Took it to apple and they found an issue with the front camera. They didn't know if that was the cause of the heat but they exchanged it anyway (instead of just replacing the front). No issues at all with my new and it never gets as hot as the one before. Not even close. So, take it to apple.
Also, forgot to mention they since I couldn't reproduce the heat on command (no idea what caused it when it happened) they said they found no problem and thus only suggested reinstall for that as well. Seems to depend on the staff at the time. But they ran all the tests and nothing turned up despite poor battery, over heating and the speak rattle I've had since purchase.