I used to be that way. It wouldn't happen in the shower or the bath, but when I was washing my hair under the faucet in the tub. I always wash my hair separate for some reason. Probably b/c it tends to get knotted up. It was never really an actual attack, but more like anxiety. I would always try to focus on something else. It usually works. I used to always feel too enclosed or trapped in the shower for some reason. You just have to tell yourself that you are in a safe place. Say to yourself, I am ok. I'm fine. I know this may sound stupid, but it works.