Been awhile since I experienced this but today my heart was going at about 90-100 bpm for a couple of hours. Even during and after a nap.
I sat at the dining table looking at both my Apple Watch and Polar chest monitor app going at 115 bpm for awhile.
I was not in any particular pain or discomfort , just the usual weird feeling. After it got back to its normal rhythm, I felt the usual tiredness that accompanies it.
I guess my heart has a mind of its own too.