today I opened whatsapp and a notification appeared about that I had to update my whatsapp within 15 days or else....
I found this very strange considering I always thought updates happens automatically from the google plays store..
When I clicked on "ok" whatsapp wanted to download a apk file.
I don't understand this at all. this is not normal, is it?

I thought I had downloaded whatsapp from the google play store and then it update it automatically right?
But now i am not sure anymore. How can i find out where i downloaded whatsapp?

Go to Settings -> Apps -> Whatsapp, scroll to the bottom and you should see "App details" and where it was installed from.

    yourmother just above your screenshot under the subheading App details it will tell you which app store you used to install it