Based on your post, it sounds like you initiated a complimentary session of Pandora Premium as the average length of your complimentary session may fluctuate. Then, you may have upgraded to a free trial of Pandora Plus.
You can find out more information about how Pandora Premium sessions work here.
You can double check to make sure that your free trial of Pandora Plus is active from your mobile device.
If you visit your settings, you can see which free trial or paid subscription you're currently upgraded to.
Click on your Profile icon in the bottom right-hand corner on your mobile device
Scroll down to Subscription
From the Subscription tab you should be able to view your account subscription status