first off : did you follow the directions on the test precisely? That's a big one - be sure to read the directions and follow them exactly as they state.
Second: did you want a baby? or are you trying to stay baby-free?
Third: I'm not sure about you, but I constantly hear women around me talking about how irregular their periods are. I've heard some say that they've gone months without them.
Fourth: perhaps your period-timing changed because of close and constant interaction with another female - something about being near other girls can cause periods to have weird shifts in timing.
Anyway, just some stuff to stew over. Sorry I'm a guy, it might be weird to hear from a Y-chromosome having individual.
*I'm not a doctor, so if you have serious concerns, seek medical advice from a real doctor*