Surgical Incision is Infected

I had my puppy spayed 5 days ago and we were back at the vet today with an infected incision. The vet put her on Simplicef 100mg once a day for 7 days. Has anyone used this? I am wondering how long before it starts to get better. It is very rainy here right now and it is almost impossible to keep her from getting her incision dirty when she goes out to potty with all the mud and water.