Low heart rate postpartum

I'm 5 weeks postpartum and started to feel up to exercising again. I've a fitbit charge hr and my resting heart rate has always been in the low 60s. After recently wearing it again, my resting heart rate has been in the low 50s, and went as low as 46. Not that you should consider a fitbit a medical device, but has anyone else's heart rate dropped after giving birth?