calculating how far along...

Chioma
I am currently 6 weeks and 3 days, if I calculate from my LMP. BUT Ihave a shorter cycle (26 days) and that particular month my period only lasted 3 days. 
Long story short, I ovulated may 11. I've been using an app for years before I even found glow and it's usually very accurate and it's giving me a different EDD.  Plus I'm sure that's when it happened because my SO and I were VERY careful before we started trying. 
Could my EDD be different? I go for my first ultrasound in two weeks. I know I'll find out then but I've been thinking about this for a while!