You might have inflamed gums or food stuck in your braces. Please floss and brush carefully

The most likely cause is inadequate cleaning of the teeth and gums. Make sure to brush thoroughly after eating and floss daily. It also helps to use an anti-microbial mouthwash like Listerine.

Braces should not contribute to mouth odor, although they make oral hygiene more difficult. make sure you are brushing your tongue and palate, not eating upsetting foods or beverages, and not mouth breathing. Thorough cleaning after meals and before bed are essential. If you do this well and still have a problem, then you may have some gastric reflux problems that your doctor could help you with. Your breath should not be noticeable unless you are smoking or drinking alcohol.

It is from bacteria. You have to brush and floss and rinse after every meal. You can use a fluoride mouthwash but the first thing is very clean teeth.
Thanks.
Dr. Price