Your only heating source was the heat pump
I would turn on the baseboard when the temps would get into the teens.The heat pump would run almost 24/7. My heat pump froze into a block of ice 3 years ago. If it happens,turn on the a/c till it melts then back to heat. I started by changing the outdoor temp sensor, not it, the computer board was bad. Didn't cost a lot, around $60.