Problem with cbr 125r 2010 help please!

So I recently brought a category C Honda cbr 125R. 2010 plate, the bike is sweet I'm in love with it, however, last week I was going along around 70mph, x6000 rpm round about, and the bike changes from 6th to 5th on its own with no clutch or anything, everything's been fine since, apart from today when it went from 5th to 4th on its own. Any ideas as to what it could be? Chain isn't too tight, I'm aware it could be the sprockets.. Anything else as to what could be causing this? Any help much appreciated. Thanks :)

There are few sites that have conversations about the gearboxes on the CBR125R