Glow changed my date!

So I've recently started using glow. It predicted that my period would start on the 1st of October but then changed it to be due on the 29th of September...I'm not really sure why it changed it. If anyone has any ideas that would be great! I took a pregnancy test this morning and it was negative but if my period is still supposed to be due on the first then maybe I took it too early.
​Thanks for all of the help!