Hi there. First off I can only guess I know how you feel-. (Its hard to point at a diagnosed anxiety patient and say "I know how you feel"- Cause we all feel differently and we all are convinced we are feeling something different. "
But I might be able to give you some calm.
I, and many in here have reported that same feeling.
And while many think my suggestion is silly, i urge you to read about it.
In short I would say your experiencing a effect from sleep paralysis.
(most will experience this once or twice in a lifetime, while others quite often)-
So you can either google that, or read a post I made on the subject where Ive tried to go into as much detail as I could.
http://www.medhelp.org/posts/Anxiety/That-sense-of-danger-as-your-about-to-fall-asleep--maybe-some-answers-for-you-/show/1536956?personal_page_id=2145000#post_6979788
In short your actually consciously aware of how your body shuts down for sleep.
Something that really is not that normal, since you normally just drift of unknowing youve fallen asleep. So imagine you could feel how you went from AWAKE to SLEEPING.
Thats scary.
Why you get this now? No idea. But seams that its more likely to happen with Anxiety diagnosed people due to a chaotic sleep pattern and a mind filled with worries.
It might also be the meds your taking thats giving you this issue.
But remember I AM NOT A DOCTOR- So dont take my word as facts.-
But read the link I gave you and see what you think. It might make you want to research further.
Good Luck, and feel free to ask anything If needed.