16 days late?

my period is 16 days late. they're always on time. the last time i had sex was june 25. in july, my period was fine. in august, i had 2 periods that were 2 weeks apart. in september, my "period" was so light, i would call it spotting. i took a pregnancy test, but i moved it before time was up, so it was inaccurate, but it said negative? i'm not sure what to do. help me?