She needs to understand that the whistle means stop, so I would get her back on the lead and take her for a walk and every time you stop blow the whistle and put a hand command in her face, make shaw she stops and sits, when she's done that give lots of love or a treat know and again. After a while she will get to know she has to stop and sit when you blow the whistle & always backup with hand command at same time.
I do this at feeding time too blow the whistle and give hand command, and when she sits give her the bowl of food, she should after time understand learn that when she doe