Doctors vs app

I went to my doctor cause it's been about 18 months and no sign of a baby yet... so they said not to trust these Apps and that I'm fertile 21 days after my 1st day of my period... but this App has worked out that I'm pretty much fertile after my period.... going for a blood test next Friday to see if I'm ovulating then according to this 21 day thing... anyone got any advice ?xxx