Oxygen level after covid recovery.

My mother had covid. She is 48 years old with high blood pressure. Did not have asthma or any history of lung issues. She had to be hospitalized and was in HDU with pump getting 15 litres oxygen support. After few days she started getting better and they reduced oxygen support and eventually stopped. Doctor discharged her at 87-90 level oxygen. At home also she fluctuating oxygen level 85-92. We called that doctor and he said this is normal don't check oximeter. But as far as I know oxygen below 94 is not safe. Is this 85-92 fluctuating levels normal for post-covid recovering patient? It has been 4 days since she has been discharged from hospital. Doctor has not given any medicines to take after discharge. I check her oxygen every 4 hours it is always fluctuating between 88-91. She is on complete bed rest, only getting up to go to bathroom. Does not feel breathless in general only a little after walking which goes away in a few minutes. Is this normal?