Bad smell from the mouth could be mainly because of systemic or oral hygiene related. Do check for food deposits like plaque especially behind your lower front teeth and other areas as well.
If Yes, book a appointment with your dental expert and get a scaling ( cleaning) done.
Other causes include systemic causes.
Next Steps
consult your dentist to find the reasons behind this.
Health Tips
brush after every meal
floss atleast once a week.
get cleaning under dentist every 6 months.