What Causes Mottling On Foot And Ankle Swelling?

My 83 yr. old mother was dx'd with NSCLC 8 wks. ago and is on 2 leters of O2. Not getting uip much, and her rt. foot has been mottled with three toes almost a bluish color over the past 5 days. The rt. foot is sore and feels "stone-bruised"; she is not diabetic. The lt. ankle has been swelling slightly over the past few wks., only, She is taking 20mg. of Prednisone for the past 3 wks.; any connection?