Reasons for late periods?

Haven’t been on contraception for a few months now and I normally have very regular periods since I started. I’m 11 days late on a 28 day cycle. I’ve taken a pregnancy test was negative. Ovulated on the 25th according to the <a href="https://play.google.com/store/apps/details?id=com.glow.android">glow app</a>. Definitely not stressing.