The scholars agreed that the menstruating woman should not fast, and that her fast is not valid if she does fast, and that she has to make up the days that she did not fast in Ramadan because of her period. What you have to do is to make up for those days, and repent to Allah for your failing to seek knowledge which led you to commit this prohibited action.

If you did not fast for a valid reason, you must make it up after Ramadan. But if you did not fast deliberately with no excuse, then one of two scenarios must apply. For more, please see the detailed answer.