the racing heart feeling could be from anxiety, thyroid or chest muscle spasms. I could be missing other causes too.

Some with TOS have gone to ER thinking a heart attack- but tests came out clear - it was severe spasms causing it.
Of course if you ever think it is a HA - do go to Dr.

I had a odd spell of racing heart beat and dizziness a couple yrs ago- but after having blood and basic tests the symptoms just went away
dr thought a very odd virus or something..

blood tests should show if it's thyroid related or not.
many with borderline thyroid readings feel better with treatment.