Someone (most likely your AME followed by an inattentive Medical Branch Administrator), has messed up, I can't see how you can have a certificate valid until September if you're due either the ECG or Audiogram in July, either/both of which are integral parts of the certificate.
None the less, it's your responsibility to ensure that you have a valid medical certificate, which you haven't at the moment, this means you cannot exercise the privilages of whatever licence the medical certificate supports.
If you rely on the class one medical for employment, you're in deep doo doo.
Under the circumstances I think I would keep quiet, and go off sick and arrange a medical as quickly as possible.
At the risk of sounding like the grim reaper, this is a bad time of year to arrange medicals as many AMEs who are practising doctors are off on holiday, although you may be lucky, or may have to pay through the nose.