Diana Ult Cooldown (bug?)

Played two or three games of Diana and multiple times in each game I would use my Q then ult to the same target and then try and ult and it would be back on cooldown, which is not how her kit works. So i ended up suiciding multiple times that I would have blown up the other champion but either couldnt ult for mobility or didnt have the damage that I knew I was supposed to have. Extremely annoying.