sabre,
The BIG question is, do you have gout?
That depends on symptoms and serum uric acid. Can you get the exact number from your doctor? It is not usual to start medication with the first attack for the single reason that you don't have enough evidence and you cannot start a lifetime of medication on a HUNCH.
Yes, it is seeming more and more common for people to report their first attack after a large weight loss.
Stop the allopurinol since I doubt that you are ready to commit to it for a lifetime and just wait and see what develops. When you are CONVINCED you have gout, then it is time to get on allopurinol FOREVER.