Can be due to anxiety and judging by your posting it’s extremely likely that what you’re feeling is due to anxiety.
It’s always there because you’re always tense - even if you think you’re not. I know because I have exactly the same feelings.
As I’m getting less anxious and more relaxed the feelings are going. Not gone by any means but I know they are a lot less than when I was frantically looking for reasons for it.
I also know this probably won’t be much help to you at the moment as no reassurance could help me for long when I was at my worst. However, as you start to accept your anxiety and get better you will start thinking the same way.
I haven’t been right for 8-9 months now but I can slowly feel myself being less tense and as this happens the symptoms are lessening. Don’t expect it to go away quickly but you will get there.