I Am 26, 5.9", And Weigh 130lbs. I Eat A

I am 26, 5.9", and weigh 130lbs. I eat a lot but try to eat healthy sometimes. I don't really exercise but sometimes I do a little yoga or squats. I recently have been visiting my cardiologist. I've worn a heart monitor in the past because they think I have vessel vegal. I went in the other day and had an EKG done and my heart rate was 39. As advised, I bought a heart monitor watch and check my heart rate about 10 times a day and it's always about 40-50. My doctor is very worried and wants me to wear another heart monitor. Could you please let me know some possibilities as to why my heart rate is so low?