Go for 100% organic cotton and if using a tampon make sure that you change it at least every 6 hours. Use an organic menstrual pad at night to help reduce the chances of toxic shock syndrome. Avoid tampons made from rayon, a rayon cotton mix, or anything fragranced.
NOTE: Even organic and biodegradable products should not be flushed down the loo! They may take months to break down, blocking pipes and polluting marine environment until they do so.