Cancer made me lose weight in general, before I was diagnosed I lost 30 pounds, not to mention the treatment you're mom is going through too, it makes you not want to eat and can make it difficult too, with things tasting strange, etc.,
As far as her bum buring, I notice if I eat foods high in fat, my excretments burn really bad, but I've had my entire large intestine removed, so there is no way to consume the acid or break it down further that is coming from my small intestine. For me, tums-extra calcium has helped, I take 2 of those twice a day, it is also an anti-acid, which helps when I excrete, I also do 150mg of Zantac OTC, they've written me scripts for acid-reduction for Aciphex, Nexium, etc., they didn't do much for me, and the copays were high so I just went with an OTC regimen that had worked in the past.
I know others can provide more feedback than I, as they've been going through this longer.
Good luck with your mom.