I have had exactly the problem. I have an iPad 3 and after the most recent update iOS9 I suddenly have a restrictions passcode that never existed before.
I have heard from several people who have exactly the same problem.
Apple refuses to admit there is an issue here. They assume we are all morons who just 'forgot'. Well I'm no moron and I NEVER has a restriciton on my iPad.
I cannot restore from a backup, because I backed up my iPad post updating. Now I am supposed to reset as NEW and start all over again.
Not at all happy with the attitude from Apple and their response to this. Not happy at all.