Plaquenil doesn't have any food restrictions, so I don't think it's interfering with what you are ingesting. However, sounds like you are only getting milk & coffee/caffeine -- either by itself, and only that, can set up some strange stomach issues. I would try a day or two with chicken or beef broth and water, no caffeine and no milk, with just small bites of a white soda cracker. In other words, treat your stomach like you've had the stomach flu.
We have had members with drug-induced Lupus - I can't remember if any of them are still posting or not.
I have had stomach/gut issues with my Lupus. Eating too much of any one thing seemed to be the most problematic. Try the broths, like I said, then try boiled or broiled chicken and mashed potatoes. Try something like oatmeal - it isn't too harsh, but you want to start getting some fiber into your system. (The "every 4 days" comment made me think of fiber -- you want things moving thru your system faster than every 4 days!)
I would definitely suggest getting away from caffeine and milk, just on general principles. Let us know how it goes.
Lynnwood, Lupus & Sjogren's Moderator, Dx: 2002DIAGNOSING LUPUS & HW's LUPUS 101
"Life is far too important to be taken seriously." - Oscar Wilde