Hello Rose,
I used to have the feeling my heart was going to suddenly stop all the time. Particularly right before I fell asleep. It was one of the worst feelings ever. I am not a doctor though, so I can't really give you medical advise. I can tell you there are nurse hotlines, usually available through ERs, that you can call for advice.
That all being said, when I experience a new symptom from my anxiety and I am not sure what to make of it, I like going to this
list of anxiety symptoms to remind myself that anxiety and panic can do all sorts of crazy things to our minds and body.
I hope you find peace and comfort today.
Best wishes.