Food intolerance or stomach bug?

[mammabing]

Sorry for the detail but click off if you don’t want to know about bowel movements!
DS who will be 3 soon has had a week of sloppy poos. They started off quite mucus-y but are now pretty wet. He was pretty regularly going once a day but has gone up to 3 times a day now. He has no discomfort, no nausea or sickness, no temp or fever, only a slight runny nose.
Is this likely to be a virus or bug? Or is it a possible food intolerance? And if it is how do we go about finding out what is causing it?

Has he been eating fine? What has he been eating?

I wouldn't generally jump to allergies if he's been fine up 'til now. Its only been a week.
Adenovirus commonly causes respiratory /runny nose symptoms and gastric symptoms and can vary in severity.

Toddler diarrhoea is also common.

I'd watch and wait but keep a strict log of other symptoms and what he is eating.
Does he have Excema/ family history of allergies?