Sorry this is happening, Karen!:( But stress will bring out any problems in a cat pretty quick. Get out the feliway plug-ins and any other Bach's remedies. ;)
The first thing I thought of was coccidia. When we brought Phoenix home to see if that would help Nakita, Nakita started having diarrhea (with blood) within the first week. When we took the stool sample, it was coccidia.
Here's a better description:
Coccidia
I'm assuming you took a stool sample in?
It was an easy fix. A pill dose each day for a week (if I remember correctly) and both cats were clear.
So, don't worry too much just yet.