normally my periods were lik clock work atleast 3 weeks apart.. now i am having one every 15-18 days and i am sexually active, i am on day 18 without the period & a negative pregnancy test. i have had cramps for two days but no bleeding?
Have you only recently become sexually active for the first time?
Depending on that, and how frequent, it's possible for you to have bleeding that mimics a period, if not an early period triggered by the new activity.
It may be a bit before your body adjusts and resettles into a new rhythym. Keep tests handy to help sate the curiosity, and see a gynecologist if the bleeding or symptoms worsen.