Your card was declined error on checkout, before adding card??

[JessNick]

Hi - yesterday I ran a simulated payment for a successful transaction, followed by a non-successful transaction in Shopify. Both worked at the time, store was in test mode. Today I've gone to run another test and the checkout page is automatically coming up with "Your card was declined. Try again or use a different payment method."  This is BEFORE I've entered any card details. When I do enter the details and press 'continue' button, nothing happens, screen stays the same with error message.  See attached.  I notice today it's auto-populated with my info from running the test yesterday. Any advice hugely appreciated as I'm about to launch my store. Eeek!