Stamina is disappearing

[7t2z28]

Twice on the same monster I've had stamina disappear.

I started on it with 40 stamina on it this yesterday evening, and after 7 power hits, it shows me out of stamina... What's more, I had a critical hit that disappeared at the same time. WTF?!?

Just now, I started with 35 stamina. I get 5 hits in, and it shows me that I'm out.

I don't get it. I attack, wait 2-3 seconds and click attack again. Is it just me the game hates or is the server stealing other players stamina as well?