In the moment, anxiety is terrifying - necessarily by it's definition. For those not too familiar with the process, here's how psychologists theorise that anxiety works:

Take an individual who is finely attuned to their bodily sensations.

'Oh, what was that? A strange twitch?'

A less neurotic person ignores it, but the anxious persist.

'Why would there be a twitch?'

Worry triggers other autonomic effects. Breathing increases, heartbeat becomes prominent, maybe some sweating or shaking. Before they know it, the anxious are suffering a full-blown panic attack, drowning in their bedrooms which have inexplicably filled with water, muffled noise and bubbles of air the only things to come out as they try to scream, "WHAT IS WRONG WITH ME?!"

So the water thing may not be an exactly common symptom. But I believe it is normal to attribute the bodily symptoms to external events. I felt a monumental pressure on my chest and I couldn't breath. Ergo, I was drowning. My brain doesn't usually let me down, but I really think it could have come up with a more plausable explanation in this case.

None-the-less, there I was standing on my bed, craning my neck to reach the surface, tears streamimg down my face (further evidence for the water interpretation). Brother opens my door, his confusion causing him to forget what he came in for, stares perplexed for a moment, then slowly back-tracks and leaves. Which makes me reassess the situation and decide that I was indeed not drowning.

Anxiety, in retrospect, is hilarious.