Physical stress reactions without obvious cause for stress

Recently I've been having a few physical signs of stress and/or anxiety without feeling stressed or anxious.

The most jarring of these are trouble breathing, increased resting heart rate, and heart palpitations. I am not taking any medications and can see no cause for these reactions, so I have been thinking about a way to figure out what may be the cause for it.

So far, the best thing I can come up with is to have someone listen to my heart while I listen to different sounds or look at different video's/images.
I just have no idea what bothers me other than being in a moving vehicle or being in social situations.

During the time these reactions occur, I am not doing anything that stresses me, and yet I have randomly experienced extreme paranoia, chest tightening, trouble breathing, etc.

I was wondering if anyone had a better method of figuring out what is causing these reactions, or if anyone can offer suggestions on what to look at/listen to while monitoring my heart.

All suggestions are appreciated. Thank you in advance to those who answer.