Sometimes new food and/or treats do not agree with them. Always if mine have diarrhea and/or vomiting I watch them closely and take them to a vet if it escalates. Many times they are just fine....just need to get whatever it is out of their system.
It is important to be sure that they are hydrated. A way to check for dehydration which can occur when they are ill is to lift their upper lip and check their gums. In dehydration, the gums become very sticky to the touch.
Another thing to watch for is lowered blood sugar. Here is a link:
Hypoglycemia Symptoms and Treatment