Many of us have had this experience.
Here are a few reasons you may feel tired:
Your body is fighting a serious infection.
You have two or maybe 3 lung diseases which deprive your body of necessary oxygen, not only when you move around, but all the time.
You are hitting your already stressed body 3 times a week with powerful antibiotics.

Here are a few things you can try - intentional deep breathing when you are sitting to get more oxygen. Airway clearance to get the mucus out of your lungs (more room for air). If all else fails maybe ask for supplemental oxygen.
How long have you taken the antibiotics?

I'm really sorry. Love my Fitbit watch!! I can only imagine how hard it is to get "walking" exercise. Could you do chair aerobics? I do know they make a walking exerciser that sits on the floor and you move feet back and forth. My mother in law got one when she was trying to gain back energy and strength after having Covid really bad - was hospitalized and didn't know if she was going to pull through.