Your surgeon's goal was to speed up resolution of swelling by injecting steroids into the cheeks. One of the steroid's effect on fat tissue is atrophy, i.e. decrease in the fat volume. This is usually long-lasting (months) but can return to almost normal volume. Sometimes, some permanent volume reduction persists. In addition, the skin can also thin out which may in part be the explanation for the purplish color you mention.
My "distant" advice would be to be conservative at first. Do not try to fix right away what you are concerned about because things are in progress and will change over time. You do not want find yourself chasing a new problem. At the end, these issues can certainly be improved upon by various means (injectable fillers, fat transfer).
Stay in close contact with your plastic surgeon as s/he wants to monitor your progress.
