I could only find a little information on dogs and nopales here:
http://www.herbalsafety.utep.edu/herbs-pdfs/nopal.pdf
It sounds like in massive quantities, nopales can reduce blood sugar, leading to hypoglycemia, but I doubt that a few small pieces would have any effect. Since she's fine this morning, she's probably out of danger, but I'd have some nutrical or karo syrup on hand for just in case.