incorrect username / password on checkout

I’ve built a new site this weekend and upon testing, the logins for users don’t work.

Initially they wouldn’t work on the log in page but I fixed that by changing the email to wordpress.

So now when i go to shop and check out – it asks me to log in (even though I only just logged in) So I try to log in using the same credentials and it comes up with an error – Wrong username or password.

When I go to the log in page to log in again – it is successful, but when I try to check out it comes up with the same error message. Help!