Welcome to the FAQ website covering document topics in Canada.
I just completed the Citizenship oath and now have my Citizenship certificate. However, I did not realise that my PR card would be cancelled at the same time. I am travelling to the US next week and will not have time to complete the Canadian Passport process. Can I just travel to the US and back in on my UK passport? I also have NEXUS.
You will likely be allowed to re-enter Canada with only your UK Passport. If you do that, bring your Citizenship certificate with you. However, the safest thing to do would be to apply for a Canadian passport using their 48 hour express service.