Thanks for posting your query.
As your mother is diabetic and chronic asthmatic, it is no wonder to get infections in such chronic diseases when body's immune is compromised. Sever diarrhea
and nausea suggests gastroenteritis
which can be viral or bacterial. Continuous diarrhea dehydrates body and may show sign of dizziness
and light headedness, So every diarrhea should be treated with water and electrolyte replacement. You may also use loperamide
You are advised to get stool analysis so that it can be treated with proper antibiotics if needed.
Hope it helps.