Well...hmmmm... sometimes you can mess up preg. test by getting to much "peepee" on them and sometimes [if your preg.] you'll have pregnancy hormones but not enough to show up on a test if you have any at all by the time you take it. Neways, When I was on my BCs my periods would mess up like that too, how old r u? If you a teen then your body might be doing that crazy hormone changeing mess and stress will sometimes change your periods too,well,good luck finding it out!