Second line magically appeared after stated timeq

I'm wondering if anyone can help... I did this pregnancy test under a week ago and after 5 mins there was only one line so I popped it in the bin.. I've come to empty it and realised that another rather thin line has appeared.. My sister seems to think I'm pregnant as this has happened to her almost every time and she's got 3 children has this ever happened to anyone and been pregnant? Xx