What could cause low plasma cortisol, low plasma potassium, with nausea, muscle weakness, fatigue, lightheaded, vertigo. not taking any medications. thank you?

I would suspect that cortisol was measured at the wrong time or that you are taking(perhaps unknowingly in a supplement) a steroid that does not show up as cortisol in the assay and that steroid is causing the low K. Salivary cortisol measurements may be better. Recommend you spend $20 for 1 week access to uptodate.com and explore the causes in detail then discuss with your team.

You need to see your MD or an endocrinologist for proper diagnosis and management.