My new kitten has diarrhea? ?

I had my lil Rio today he didn't poop/wee all day but has done both now so I was happy but then I saw that he'd been sick on the mat then he went in his tray and had diarrhea! I have fed him kitten premium quality dry cat food and Felix kitten wet food (as instructed by rescue shelter) what's the matter with him? He's acting scared and runs away all the time now? He's been so playful all day? What's happened? X