Hello @lola1971 and welcome to Mayo Connect. As I was unfamiliar with SIADH, I went online and found some information from WebMD. Here is a link that explains it for others who might be interested.
\https://www.webmd.com/a-to-z-guides/what-is-siadh

Could you share a bit, as you are comfortable doing so, what symptoms led to this diagnosis and how you are feeling now?

Lola, I think you need to be certain that the doctor prescribing the z-pack (azithromycin) is aware that you have SIADH. What illness was this prescribed for?
Reading the article
Also, most of the people posting in this particular discussion are taking antibiotics, especially azithromycin, long-term (one to two years) for an difficult-to-treat infection deep in their lungs.

Here is an excerpt talking about antibiotics and SIADH from the NIH:
"Several medications have been implicated in medication-induced SIADH, but antibiotics are rarely on the list [5]. There are a handful of case reports implicating trimethoprim-sulfamethoxazole, ciprofloxacin, cefoperazone/sulbactam, rifabutin, azithromycin, and nitrofurantoin. However, these have occurred in elderly individuals, in the presence of other comorbidities, and concomitant use of diuretics and antidepressants [6]."
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10105516/
In your situation, I would probably get my sodium checked after you finish the antibiotics, unless you feel symptoms this weekend. In that case, since it is a holiday, you may need to call your clinic's emergency number, or visit an emergency department for an IV.
Please let us know how you are feeling.
Sue