Get your iron tested (anemic-same issue!)

Are you really athletic? Athletes tend to have lower heart rates in general because their hearts are more effective at pumping blood, so they don't need to do it as often

Maybe check for POTS. It’s common with fibro

Make sure you’re eating enough! Same thing happened to me while training for an Ironman and I was accidentally under fueling! Healthy fats for women are always good :)

Likely your heart is just healthy! Can run some labs but not abnormal for those rates if you’re young and healthy.