My cat is not acting normal?

This morning I noticed my cat was ot acting herself. She was sitting in the bathroom which was unusual and when I pushed her to leave she was hesitant to move. She has not been eating, not taking treats. She just sits in one spot and kind of grumbles to herself. Normally she is outgoing, purring, and jumping up to see me. The stool looks normal, and the food is nothing different than what she has been eating. Do cats have an upset stomache sometimes? She had a lot of little treats yesterday. She is an indoor car, there is nothing that could make her sick. Will this pass or should I take her to the vet? I am only asking this because a vet trip could easily become $300.