What causes sulfur burps associated with lack of appetite, mild nausea, abdominal cramping, and bad gas? (not pregnant)

Could be Giardia: Giardiasis, a parasite, lives in the upper small intestine and classically causes "sulfur burps". Usually contracted from drinking non city water - for example on a camping trip. I'd start with a stool exam for giardiasis (not the traditional microscope exam but what is called giardia antigen, an immunologic test on the stool).