Hello, thank you for consulting with healthcaremagic.
This foul odour comes from mouth when there is infection of the gums.
Actually when the teeth are dirty there is formation of plaque
over the the teeth and this plaque later on causes inflammation of the gums which leads to this foul smell.
There is no need to worry you just visit a dentist
nearby and get the teeth cleaned, and visit him regularly for the follow up.
Hope it will help you.