On one hand, it could very easily be 水土不服 or just not used to the style of food the Mainlanders eat. But on the other hand, you can't really rule out the possibility of improper handling of food, fake / poisonous food, industrial additives, etc.
Aside from exercising your best common sense -- ie. going to more established and/or cleaner-looking restaurants, not eating dubiously cheap street food, etc., there really isn't much you can do. IIRC, even well-established chains such as 真功夫 was involved with some questionable food supply / safety issues a little while back. And then there were reports of Park'n Shop (百佳) in SZ or GZ selling "beef" made from "pork marinated with beef paste". 
When you can't tell the fake and the real stuff apart, the only sensible thing you can do is to assume that everything is fake. Otherwise, you might as well just not give a damn and pretend that everything is real, cheap, and tastes awesome because they just naturally are. 
-Lik |