Liz Buchanan BVSc MRCVS
Hello! You say that your vet was under the impression that Archie was fine and ready to go home, but that you are now observing lethargy, weakness and lack of interest in food. This sounds like a definite change in expectations, so I would advise calling your vet, or emergency vet (I don't know where you are but it's Sunday night here) and giving them an update, just in case this information changes what they want to do in the short term.
A price of information that might help your phone triage, if you can do this confidently, is to count the number of times Archie's heart beats in a minute (I'd usually count this over 15 seconds and multiply it by 4). However if you are not sure about this, always admit it (untrue numbers won't help).
All the best and please do let me know how Archie gets on. Addison's secondary to Cushing's treatment can be serious, and potassium levels can change quite quickly, so the emergency vet may want to rule this out.