Don't worry about it! Sometimes happens when you've signed up and use the app and request a Premium trial for the first time. There you'll receive the 7-day trial but if you've already signed up for the 30 days, your trial will probably extended after the 7 days.
If this doesn't happen after the 7 days, then I'd suggest checking the Spotify web page and request once again the 30 days trial!
Keep me informed on how things go so I can assist you if something happens!