You should try deleting and reinstalling the app. Then, login into your Spotify profile on your browser here with your email. Your username will be displayed after you login.
After that, check the status of your subscription. If everything looks ok, logout of the app in your device, and be sure to use that username, instead of your email address, to login to your account. It's the one with the Premium features.
Hopefully that'll fix the issue. 🙂
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