What questions should I ask the oncologist about low oxygen levels?

My 88 year old mom was diagnosed 20 months ago with stage IV Adenocarcinoma. There were a couple small spots on one lung and a pleural effusion. No other spread. She has been doing ok except for her appetite and she has lost 60 pounds. Apparently she was not a candidate for chemo, only Keytruda immunotherapy every 3 weeks. Wednesday she had her infusion, took a nap, and woke up gasping for air. She went to the ER, labs were good, they did a thoracentesis and sent her home. Before being sent home her oxygen sat level was 86. She has not needed home oxygen, so the ER ordered it to be delivered on Thursday. We were surprised that they sent her home with low oxygen and did not give us a home oxygen tank for Wednesday night.

Since Wednesday night her oxygen has been fluctuating. Since we received the home oxygen, while sitting, with the machine at 2.5, her oxygen reads between 89 and 93. When she gets up, it quickly drops to the mid to low 80s. We’ve been advised to turn the machine up to 3 or 4 when she gets up. No one has told us that this is her new normal, they just tell us if it stays in the 80s to go the ER.

Does anyone have experience with this? Does this signal the end is near or warrant further testing? I’m have posted here before that Mom likes to present a pretty picture when talking with doctors. She had a phone appointment with her primary care doctor yesterday, told her that while sitting her oxygen is 91, that she feels good. When she gets off the phone she is anxious, checking her levels constantly, panics when she gets up and has to gasp for air. It is very frustrating to say the least. As her caregiver I don’t know how to help. She gets angry if I ask the doctors questions. I just am at a loss in trying to help her.