Annie,

There is no provision for such an exception.

In the Latin Rite, the rules today are so minimal — (abstinence from meat for one day a week: Friday; and Ash Wednesday, and fasting on Ash Wednesday and Good Friday) — that it almost certainly won't have an effect (especially abstinence from meat).

The rules used to be fasting every day and abstinence from meat every day. People lived over thousands of years like this with no effect.

If you are concerned, please consult your doctor, since this is, in part, a medical question.

If your doctor says you cannot fast on Good Friday (assuming your period coincides with Ash Wednesday or Good Friday), or that you must eat meat on each of the six remaining Fridays, then you would have an excuse.

By the way, Annie —

It's useful to review the rules in the Latin Rite for fasting.

Fasting consists of one meal during the day, effectively dropping two meals, assuming the usual cadence, although you can have two light meals not adding up to a single meal.

So basically, you're obliged to drop a meal and a half on Ash Wednesday and Good Friday.