I have read about similar case like yours, when a younger woman after withdrawal from birth control pills, have started experiencing a fecal incontinence for a year only during a week prior the monthly menstrual bleeding.
The cause for this was found to be different progestin levels during menstrual phases affecting the tone of anal opening closure muscle.
In her case, symptoms resolved after starting the use of birth control pills for second time.

See abdominal surgeon for complete evaluation!