There are a few possibilities, one -that she has an infection
.....two, that if it is only still a few days since her spaying, she could still have some throat
irrtation from where the breathing-tube went in for her anesthetic.
A "chesty" cough always needs to be evaluated by the vet, I think. It may still be residual irritation from her anesthetic etc, (if that was fairly recent) or it may be something else which needs treatment. I would take her back to the vet to be checked.