There was a small change made to the OAuth process last week. You can read more about it here: https://ecommerce.shopify.com/c/api-announcements/t/oauth-api-endpoint-changes-to-signature-verifica...
Are you able to install any other apps in your store? They all use OAuth, so this can help you determine if there is a problem specific to your store.
Normally you could receive the login page if you made a GET request by mistake instead of a POST request.