Low oxygen after knee replacement.

She had total knee replacement for both knees since she had osteoarthritis 4th grade.No pre-existing condition of diabetes,heart prior to surgery.Is overweight though & has thyroid and hypertension for which the medicines were continuing. he surgery went fine & she was shifted to general ward within 24 hrs of surgery after being in icu for observation.There were no complications till then but the next day her knees bled & she had problem breathing & chest xray was taken (attached) which shows mild pneumonia.Right now, she is on supplementary oxygen but her oxygen level ( pulse oxymeter) dips to 60-70 as soon as she's off the oxygen. Although the doctors say that she's recovering and she does seem fine other than the fact she's unable to get up to urinate and defecate. Also, there doesn't seem to be any neurological or cardiac condition Is the oxygen dip normal or is it something to worry about because as per my understanding,post surgical pneumonia can have very adverse effects ?