Ive been on birth control for the past 3 years. Loestrin 24 Fe the whole time. lately ive encountered a strange period schedule. i know im not pregnant because ive taken several tests, all saying negative. ill get my period one month and miss it the next, then have it the next month and miss the one on the following month. i dont know what could be causing this because i take my pills every day at the same time, and i havnt missed one in about 2 months. What could be causing this?