Let me try to solve the above describe issue,
The side effect no doubts of chemotherapy
may cause stomach upset
but the chances of still persistent side effect are negligible , I mean to say that mostly all the side effects are subsided with in the 3 months of stopping of therapy, and it has been way more then in your mother's case so "NO" mostly its not a from the side effects because the treatment was taken way in past.
NOT LETS ADDRESS THE PAIN IN STOMACH
following seems to be one of the cause for the issue,
1) Viral gastritis
2) H. Pylori infection
3) Gastric ulcer ( If pain occurs with empty stomach/with food intake)
4) Duodenal ulcer
( If pain occurs 1-2 hours after the meal)
5) GERD/ACID REFLUX
Here is the mostly will do
1) will do some physical exam
2) check for H.pylori infection
3) may do an endoscopy
to see the ulcer
4) may do barium swallow x ray
to find some other underlying issues
a) Antibiotic for the H.pylori infection
b) In case of ulcer , may prescribe some medication to heal faster/ may do some endoscopic procedure to fix it
c)For- Acid reflux they may add some other acid blocker which may have better result.
Hope it helps.