Did you happen to enter in any card payment details on your account recently? The 7 days free premium is an option on mobile devices, and when it runs out, your account should revert back to a free ad supported account. The nice option here is that users first time premium users can take advantage of both free premium offerings by first accepting the 7 day free premium on mobile Spotify apps, and then signing up for premium on your Spotify account page by entering in card payment information on the user account page here: https://www.spotify.com/account/subscription/ users can get 37 days of free premium by accepting both offers. Also when signing up for premium pay very close attention to the sign up offers on the more offers page here: https://www.spotify.com/purchase/panel/#__more-offers this is the holiday season, and Spotify offers sometimes 3 months of premium for .99 cents for first time premium subscribers who sign up for premium services with card payment details added to their accounts. This 3 for .99 cents is for subscribers who have not had premium subscriptions before on their Spotify accounts.
By going here: https://www.spotify.com/account/overview/ on your account page, look to the right of your Profile information and you should see a section that has information on your account level by the words of Spotify Premium with a check mark next to the left of those words. Also below this will be an exact date that your Premium will either expire, or renew if one has added a card payment details to auto enroll with premium from month to month, or if one has purchased a full year of premium, any case there will be a date when premium will end or the next billing cycle begins on the account either way.