Well, she doesn't look like faverolles.. (thinking of breeds with puffy cheeks, but she only has 4 toes and clean legs)
I was just thinking some of the larger heritage breeds like them grow and develop sooo much slower than other breeds, in my experience. So I would scour their site and look at their chick pics until finding one that looks similar. I have gotten white EE and a beautiful girl she was, as well as some have definitely been on the smaller side and lower in the pecking order. Including check their bantams, though my bantams never have that large of growth difference that early on... all are individuals and the more I see the less surprises me.
But that doesn't fully explain her behavior.
Being on medicated feed and vaccinated mean the chick should have an easier time fighting them if exposed, but does not mean absolutely won't get them. So just don't automatically rule them out if you do start to see symptoms.
That being said, every chick poo has coccidia in it. The medicated feed has amprol *usually* which act as a thiamine blocker to slow the growth of coccida at a maintenance dose. But sometimes you may still need to treat if conditions allow for the overgrowth, usually weather or brooder related. Only 1 out of 9 cocci strains will present as blood in the stool. If you wanted to be extra sure that wasn't your issue they can run a fecal float at the vet. Mine cost about $15. I'm not saying that's it, just sharing info. Hate to be caught of guard by a false sense of security.
I apologize if I repeat any info I've already given you. Gets hard to keep all the threads straight sometimes.