Although many factors are involved but Bad odour or Halitosis is the mostly found in oral cavity with calculus deposits. Acidity could be another reason.
Next Steps
Visit a dental Clinic to rule out exact cause. If calculus deposits are found then ultrasonic scaling followed by a course of medicines and mouth wash will solve your problem.
Health Tips
Make sure you follow correct brushing technique. As doing brush 2 times a day is good but correct strokes of brush will keep tartar away for long.