Flash Disabled by some interaction by Thresh

So I was playing a Normal game with pals and I was chasing a Thresh. He then proceeded to grab me with his Q, then I found that I couldn't Flash and Flash was greyed out. I even looked on the scoreboard and asked my buddies to check if it was up, and sure enough, it was still available. I just couldn't use it for some reason. It wasn't until later on in the game when he hooked me again that I was able to use Flash immediately afterward. Not sure why this was. Sorry if this description wasn't too helpful.