Oxygen Saturation. Why this drops?

My mother had covid positive n bilateral pneumonia on 31st March and get discharge by 13 april, as per treatment the stroids already given in hospital and according to prescription I also given medrol 16mg trice for five days than 8 mg than 4 mg for 5-5 days, than again dexamethasone 8 mg once daily for 7 days and 4 mg for 3 days and stop Rest other on going medecine are Perfenix 200 thrice daily Cardace protect 2.5 once daily Tab doxofylline & acebrophylline 400+100 once Tab (Montelukast+ fexofenadine ) one tab at bedtime Foracort 400 inhaler twice Tiate inhaler once . Mucinac 600 twice Calpol 500 twice Saturation maintain 88-92 without oxygen support with no difficult, but while walking and going to washroom saturation drops to 75 than again back to 90 around. Why this drops? Recent report on 23 May like frettin d dimer nrprobnp all ok except this Crp is 21 ESR is 37 Sir/mam Question is Saturation proper maintain kab hoga.