It could be related to a reaction to something in one of the meals/snacks from Nutrisystem, or more likely it could be a bout of stomach flu which is not really a flu. It's proper name is viral gastroenteritis.
Usually gastroenteritis resolves within a few days.
You should suspend the Nutrisystem program for now, stay hydrated and follow a bland diet-dry toast, rice broth or plain
rice, bananas, applesauce also known as BRAT diet (acronym).
Avoid greasy &spicy foods, dairy, fresh fruit and raw vegetables as well as alcohol.
Taking a drink with electrolytes is very important.
Coconut water is a good option.
There are other possibilities of course, but the aforementioned
stick out more.
If your symptoms are severe and persist, you should see a doctor for a medical evaluation.
Best wishes,
Niko