Putrid odor coming from bandage when it is removed

When changing the dressing on my mother's foot,there is a very nasty odor that comes from the bandage. She has had surgery for a diabetic ulcer on her heel,they removed part of the bone from her heel to help the ulcer heal,It has been doing very well with the surgery and the artificial skin graft. wWe have been using Regranex for the last 3 weeks now,and only the last week it has had a very putrid odor when I open the bandage. I don't know if this is common or not. The heel is not red or swollen. There is a little yellowish to light brown discharge on the bandage every night though. Is this something to worry about. I don't want anything setting my Mom back again.