Head to the Membership tab in the app, here you’ll see all the credits available to you with your membership plan. For each credit you'll see how much is left, what it can be used on, the minimum spend, and the date it needs to be used by.
You can also see what you've already spent it on, so you can check where a credit went.
Anything you haven't used by the expiry date is lost. Rewards credit doesn't roll over into the next period.