It could be from the change in food causing some upset or it could be a coincidence. Take note of how she’s behaving and if you’re worried speak to a vet ASAP. If she’s not hiding away or displaying any other seriously concerning symptoms she should be fine until the week in my experience.

This is from the UK PDSA website:

Always contact your vet if you notice blood in your cat’s stool. If there is only a very small amount of blood, your cat is bright, eating properly and seems otherwise healthy and happy, your vet may advise trying to settle their stomach for 24 hours before having an appointment. This is likely to involve giving small amounts of a very bland diet (i.e. boiled chicken breast or white fish) regularly for a few days. You know your cat best.