Got an extra period this month, on day 17 of cycle. would the dexamethasone i took for suppression test that week have caused this?

Unlikely: The Dexamethasone test should not have affected your menstrual cycle, ???Implantation bleeding?? Could you be pregnant??? Hope this helps Dr Z

Generally not: Hi. The standard overnight sex suppression test uses a one-time sex dose of 1 mg. I would not expect any effect on your period (or to cause an extra one), but never say never. If you DO have Cushing's, that generally doesn't throw in a lot of EXTRA periods either.