I had a really fast heart rate when I was pregnant and I was struggling to breathe when walking up stairs etc. Turned out my ferritin levels were incredibly low and I had to have an iron infusion. You could ask for your ferritin to be checked via a blood test
My heart rate was consistently over 100...even when resting
Can you get a device to help at night? I had my 21w scan last week and they want me to come in at 32 weeks to make sure the baby is getting enough oxygen due to my asthma. So I would say ask your doctor and see if there is anything that can help.