Babies and the runs?

My 14 week old baby has had spouts of diaohrea (sorry i do not know how to spell this word) it smells of fish, and has a lot of mucus in it, i did not think to ask my GP as i thought all babies poo was soft and smelly. but surely this isnt the same thing? I will go to my GP in the morning but for now a most likely answer? It has happened three times today where as usually he will only 'go' once a day.