Is she considered sinful if she delayed the Dhuhr prayer, thinking she would not menstruate, then found out she was menstruating immediately before ablution?

If a woman predominantly believes she has menstruated during the prayer time, she is obligated to promptly perform the prayer. A mere possibility does not justify delaying it. There is no sin upon you if you do not predominantly believe that menstruation has occurred. It is recommended to make up the prayer if menstruation began during its designated time.