First, I think thyroid. That can surely do it.
However, we found out that I had an auto-immune disease when I put on 20# in a week after eating literally nothing and drinking just water in that time period. I was heavy, and was at the borderline of developing an eating disorder. All of my excess weight was water, and it was due to proteinuria, or protein being spilled in the urine. Without protein, water floats around your body at the will of osmosis and gravity. My kidney function was on the decline due to scarred glomeruli (filters).
However, it was unusual for them to find it in me as a teen, as my disease is normally diagnosed in early childhood. It is unlikely in most adults.
Still, I would also seek renal function - GFR, creatinine, BUN, and even a 24-hr urine panel. That last one is tedious, but accurate.
Are there other factors like drugs or alcohol? And a person can insist that they aren’t overeating and maintain control around family, but when alone, will binge. That is an eating disorder, and I was an expert at hiding my behavior when I was nearing ED territory.
The only way to sort it out either way is to be admitted to a hospital with a controlled diet and fluid intake monitored - I surmise they’d do that anyway considering they don’t know what’s wrong.
Also, can confirm that pharmacists are excellent resources you don’t have to pay to speak to. Granted, most will say, “Dude, see a doctor!” with something complex, but they’re almost doctors when they’re done with school. I’m a pharma tech, and I have been fortunate to have very knowledgeable pharmacists as a patient AND as bosses. I work at the pharmacy I use.
They can, legally, go over meds with the patient and point out any interactions or drugs and supplements with odd side effects.