Hi Sandra,
Not finding answers is not only frustrating, it can be scary, especially when the symptom is a rapid heart rate. While we will have to leave the diagnosis up to the medical professionals, I appreciate your question of "where to go next?" I'm tagging a few member who may have some thoughts after reading your message: @tomonthebeach @vdouglas @predictable @jigglejaws94 and @loli for example.

You may wish to view this discussion on Connect, “What could this possibly be?” https://connect.mayoclinic.org/discussion/what-could-this-possibly-be/
I also invite you to follow the Autoimmune Diseases group https://connect.mayoclinic.org/group/autoimmune-diseases/
Sjogren's https://connect.mayoclinic.org/discussion/sjougrens/