Blood in stool, unsure what to do

I found a small amount of stool on the floor with a quarter-sized amount of bright red blood. It could be one of two cats....

First, I don't know whom to call. It's Sunday, so the only vets open will be the emergency vets, and from what I've read, bright red blood in stool is not an emergency. Still, I feel like I should call a vet for advice. I guess I could wait until Monday.

Second, I'm not sure that there's enough stool/blood to have it tested. And even if there is, I assume it would not be "fresh" enough to take in on Monday? I guess I would have to take it to the emergency vet, assuming they have a lab there. Is it worth it to get a test when this only happened once?

I already know a likely cause: we had to switch foods abruptly without transition. Same brand, slightly different variety, and both cats in question have sensitive digestive systems. Not good, but I had little choice.

(Come to think of it, we did have this issue once before - just with significantly less blood - and my vet said not to be concerned unless it persists. This time there is more blood and I am concerned.)

*An update - I just saw one of the cats straining to defecate. I would think this probably caused the bleeding....I'll keep an eye on him.

If you can get a fresh stool sample this afternoon or evening, store it in a ziploc type bag and place it in the fridge. Call the vet and/or take it to the vet tomorrow for analysis. You could possibly call the ER vet and find out what they recommend you doing tonight. It's possible that your kitty is constipated, but it could be something else entirely.