what is anxiety and what is this ?

All the symptoms you describe can be associated with anxiety  which takes many forms and can produce symptoms even when relaxing as anxiety can be triggered by things at a sub concious level, it may be something you saw on TV even. This doesn't mean you will feel anxious all the time it can come and go depending on what stresses you have in uour life.

Anxiety is the bogies fight or flight response and is highly tuned and under normal circumstances is a healthy response to danger however it can become an illness if it is yriggered at inappropriate times due to perceived danger which isn't there. Anxiety is often associated with depression as they often go hand in hand with each other.

Regarding heart beat if the BPM is normal then that's probably not anxiety, but you hearing or feeling it is likely to be anxiety in so much as you have become "aware" of it beating, as normally you would tune it out and ignore it. Pain in arm etc could well be an old mattress related but also anxiety as the response is to tense up musvles ready for action so they do ache at times, especially arms and stomach.

You can be sure that you are okay as you have had the tests and all clear, anxiety can be treated effectively so I would recommend a visit to your GP for referral for CBT which is a talking therapy which will focus on identifying triggers and dealing with the anxiety response when it isn't appropriate to feel it.

In meantime take care of yourself, relax and get that old mattress changed! Lol. Neil