You can fast during PMS/PMT and your period, it just makes the fast a mite more challenging! :) You can drink 2 cups of parsley tea to act as a mild diuretic, and if your period hasn't actually started yet, drink dandelion tea as a mild diuretic and liver cleanser, which ultimately helps PMS.
I don't recommend the use of herbs that have an estrogen hormonal action on the body for menstruation management except in the case of emergency such as a super-long period that lowers your hemoglobin. From everything that I've read, monkeying with estrogen long-term causes long-term problems.