Generally halitosis/ bad breath is due to presence of tartar/ plaque which is the reason. Removal of this by a procedure called Scaling/Cleaning/Oral prophylaxis every 6 months will relieve you of this problem.

Smell from mouth may be due to many reasons. Knowing the cause is very important. 1. Bad breath may be due to tartar deposits which is not removed even after proper brushing. You will require a professional Dental cleaning to get rid of deposits. 2. Dry mouth can also cause bad breath. 3. Some medical problems may cause bad breath like stomach or liver problems or throat infection. 4. Food like onion & garlic may cause bad breath. 5. Infections in mouth like pus etc may also cause bad breath. Get yourself checked for any of these causes & take the appropriate treatment.

There are many causes for halitosis (bad smell from mouth). According to your question foul smell after brushing may be any carious tooth present in mouth that causes food lodgement in the cavity, may be GIT Problem, may be you have smoking habbit. First of all you go to dental clinic and discuss your problem with dentist. If your teeth alignment is irregular it may be cause for halitosis.