CD 34

I've never missed af except when I found out I was preggo with both my kids. For about the past year, this app has always been right on time as to when I'll start. I've always had 28 day cycles like clock work, now I'm on cd34. AF still hasn't shown & BFN last night(5-18-16) & this morning(5-19-16). Don't think it's from stress, but no other idea of what it could be. Somebody plz help