All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
One is considered a martyr resulting from death due to an abdominal disease if he dies from diarrhea according to the definition of An-Nawawi ; out of ascites or flatulence according to Al-Qaadhi ; suffers from a severe ache in his abdomen; or dies as a result of any abdominal disease in general.
Being so, your mother could not be a martyr resulting from a death due to an abdominal disease. But, of course, this may not forbid her from being a martyr or elevated, by Allaah The Almighty, to the rank of martyrs by virtue of some of her deeds. It is narrated that the Prophet, sallallaahu ‘alayhi wa sallam, said: “He who sincerely asks Allaah for martyrdom, Allaah Elevates him to the ranks of the martyrs even if he dies on his bed.” [Muslim]
Allaah Knows best.