Popcorn ceilings are very popular in our area of the country. While you can buy a product for touching up these type of ceilings, you risk making the problem worse since the wetness of the product make cause the existing "popcorn" to come off.
What if you first spray the water spot with Kilz and then spray with a flat white paint while feathering in around the spot, i.e., spraying lightly from the spot outwards?