I just had this happen to me, Jagex likes to do things like this to give players a heads up about a future update. It also causes alot of talk about what happened. My guess is, that new skill that's supposed to come out soon, that was the start of it...

The sound of what it says depicts something similar to a stat drain, but when checked, no draining is found. Jagex have yet to announce what happened.

if you look at recent updates on the forums there are lots of threads