How long to wait for dewormer to work?

[driver8]

So we adopted a kitten from an animal shelter on Friday.

He's 11 weeks old, and was in foster care from 10 days old until 9 weeks, spent a week living at the shelter but was in a condo area with his siblings there. They had no problems with his health at the shelter.

They gave him deworming medication just before giving him to us after we signed the paperwork. It's yellow liquid. He seemed okay for the first day - poop was soft, but not runny.

Now for saturday and sunday he is having some diarrhea (not lots but about 3 or 4 a day, small), and it has some blood in it.

He's otherwise ok, TONS of energy, no other symptoms, no distress around using the litterbox (I watch). We kept him on the Iams dry kitten just because that is what they feed at the shelter, and we're not making any changes yet. He is quarantined from our other cat in a separate room.

I know that diarrhea can be a side effect of the deworming medications and also as they expel the worms. We're calling the vet first thing tomorrow, but does anyone know how long I can expect the side effects after deworming to last?