What can cause excessive gas with really bad smell?

Various things: This can be with seen with change in diet or a food intolerance such as lactose intolerance. If there is accompanying foul smelling stools/diarrhea, then causes such as gluten sensitive enteropathy (celiac disease), irritable bowel syndrome, various infections of the GI tract, or IBD (Crohns and/or Ulcerative colitis) should be considered. This a relatively non-specific symptom.

Malabsorption: Probable mal absorption of food. Most common mal absorption is for diary products. Try lactaid and see the response, You can try probiotics to replenish the gut bacterial flora.