It seems to happen only to monks. When i log-in, a weird black bar shows next to my character bar. It fades out when i change my spec and shows again only when i relog.
It might be the stagger bar?

It was the stagger bar though is currently non functional. appears in odd places and times on all monk specs atm

I haven't seen this recently while tanking on my Monk, but will keep an eye out for it. If I don't see it anytime soon we can probably close as CNR.