Oxygen Level Low After Surgery

My mother (68) had gallbladder surgery and had to stay in the hospital for 2 weeks due to her oxygen level never returning to the appropriate amount.  In fact, she just went home with oxygen yesterday.  She had a catscan 3 days after surgery - no blood clots and two days later she passed out while trying to walk in the hospital from what I was told she had several blood clots in her lungs after another catscan.  At one point it was thought may have had the clots before the surgery.  Earlier in the year she did fall and hurt her knee which required surgery - her complete leg down to her foot  was black and blue and swelling in her foot and knee. However a catscan in July did not reveal blood clots or problems.  Anyway, how long is it expected for her to be on oxygen? What could have caused this? Is there something else that maybe wrong?  She had no previous symptons of breathing issues.   She was a smoker up until about 17 years ago.