Ok, so I’ve been on BC pills ever since I had my daughter 7 months ago and never missed a pill. The start of my last period was Sept. 13th-19th. Well, on my <a href="https://play.google.com/store/apps/details?id=com.glow.android.eve">period app</a> it says I was suppose to start on Oct. 11th. This app is almost never wrong. I took one pregnant test yesterday and negative. What could it be?