During period, your body experiences a lot of hormonal changes in order to cope with the changes your body is going through. One of the hormones that cause some alterations to your gums and other mouth tissues, making your mouth a more conducive environment for these foul-smelling bacteria.
This causes dry mouth which leads to bad breath.
Another explanation is that when there’s a dip in your estrogen level, there is a reduction in your mouth’s saliva production. As a result, this can cause dry mouth which prevents bacteria and dead cells to be removed