She can't Q,R nor B
If she dies, she's able to cast these again, but if she R's then she can't cast them

I can vouch for this. It happened after I got a kill. The enemy Katarina had no kills and could still use Q and R, however, as soon as I got a kill, I was unable to do anything of the kind.

Perhaps it's to do with her passive?

Look at the 6 other threads that have already reported this? ._.

When I looked at the front page I didn't see anything. Whatever though, same thing.
And you should focus on the problem and not on how many times it's been posted