I downloaded the clue app (plain white box with red swirls so its not obvious if someone were to look on your phone) to track my cycle - I recommend people start doing this too
it tracks other things too like sex (protects, unprotected) sex drive, heavy days, light days etc and estimates when you are next due based on previous data you have entered - it also can warn you when pmt is due, when you are likely ovulating, notify you if you are late etc
very useful for the modern woman