A spayed female dog should not be able to go into heat. Occasionally a piece of an ovary will remain in the abdomen after the surgery and then may cause heat like symptoms which include a bloody vulvar discharge, nipple swelling, and nesting behaviors.
Have your veterinarian check your dog while she is displaying the symptoms.
I am trying to get some answers myself on this topic. My female spayed dog is being "chaced around" by our male dog and he is trying to mate with her. This happen several months back and I am very curious as to what is going on. Is she still releasing these hormones???