I think if she had a blockage then you wouldn't see anything coming out. Have you changed anything - food, treats, anything at all? Definitely have the vet check a sample.
Like humans, dogs will have different reactions to different foods. Is this the same food she's been on forever? It's not new for her?I am in the middle of a bag of dog food and my JRT eats the same food, so it is probably not the food.