This is an accepted solution.
By default, the system is set up to send you an email to notify you that your trial will expire. However; you may receive a prompt in your admin or a separate email that provides you with the option to extend your trial. If you accepted this offer, then you can disregard the email you received about your trial expiring.
If you haven't selected a plan, then your total trial time should appear at the bottom of the page when logged into your Shopify admin.