You recently bought a 11 week old kitten from RSPCA and she has diarrhoea with blood in it for 2-3 days but other than that she seems healthy and playful?

by the sound of that it seams she has worms but I'm not completely shore if i was you i would ask the RSPCA if they have given her a worming tablet and if they haven't visit your local vet and ask for some.