If it is a concern, I'm sure you will get to the bottom of it. You are doing the right thing by having tests done by your cardiologist first.
If everything turns out OK in the heart department, you might want to re-evaluate your anxiety situation. Are you being following by a psychologist or psychiatrist? Have you been evaluated for depression that could be aggravating your anxiety?
Another thing that may be going on is hormone levels that start changing for most women when they are in their 40's. The "perimenopausal" time of life could possibly exacerbate any heart problems, or anxiety problems.
And it is very possible for mind to work overtime when you sleep. Everything that is worrisome to you during the day will follow you into sleep, causing odd dreams and a continuation of physiological response to anxiety.
Just take it one step at a time. Right now, you are dealing with your tachycardia. Your Cardiologist will know what to do.
When you get done with the heart issues, then follow up with your anxiety and the possible causes for problems in that department.
You know, things like jigsaw puzzles, crossword puzzles, complex games, are great at taking your mind off your troubles and anxieties. You need to find something to do that takes all of your concentration so you can't think about anything else. For us anxious people, sometimes "relaxing" only gives you more time to think about the very things you don't want to think about.
Just remember, you are OK. You are actively seeking treatment. This is good. You will find the problem with the help of your doctor. Just be patient. Like my husband always tells me: "Everything will be O....K.....!"