Do it ASAP https://www.canada.ca/en/immigration-refugees-citizenship/services/application/application-forms-guides/imm5476.html
And submit it via web form https://www.canada.ca/en/immigration-refugees-citizenship/corporate/contact-ircc/web-form.html
It might be 2 different requests. For the first one you need to provide documents via web form. For the second you need to update your existing documents.
First don't try to blame them, accept your responsibility and admit your mistake, this way you will have much better chance to get an extension for your medicals.