Staffy has upset stomach it lasted for 2 days runny with mucus.

We have had our rescue staffy for 7 weeks she is 15 months old. The last 48 hrs her poo is runny and has mucus in. She seems ok in her self maybe a little more tired. She was eating and drinking normally. I have been told to fast her for 24hrs which I am doing but she won’t drink water now should I go the vet tommorow? Thanks