My dog has diarrhea...HELP 911!!!?

My Yourkie Terrior has the runs... What did I do??? I've had him for a month... On Tuesday I got sick so I had someone pick him up and he was gone Tue. afternoon, all day Wed. and Thur. morning untill 3:00pm or so. About an hr later he gets the attack of the runs.... So I called the perosn who took care of my 4.5 month old puppy and asked her if she fed him anything besides his dog food... She said no, but that her son had droped a cookie and my dog took a bite out of it. I dont thingk that would make him get the runs... What do you think? What should I give him? I herd PEPTO-BISMOL is good... Sould I try that? I feel so bad for my doggy, he cant even hold it in... =o(