Why does the glow app say I'll start my period in 10days

Brett • Married with 3 daughters living in sunny california ❤️💋💦
I'm not due to start my period till the 21st or 22nd but the <a href="https://play.google.com/store/apps/details?id=com.glow.android">glow app</a> is saying I will start the 17th. It's wrong! Has this happened to you?