Hello good evening,
This might be drug induced gastritis and ulcers in stomach and duodenum area.
You got done surgery for cancer lung and supposed to be followed by chemotherapy
or radiotherapy which you have not mentioned.
So by chemo or radio, you got usual adverse effects of them.
Chemotherapy cause sluggish bowel movements which cause ultimately constipation
and radiotherapy cause direct impact on local area of stomach and adjacent to it.
x-rays are useless investigations for your symptoms.
You can go through UGI Endoscopy and some antacids and PPI along with laxatives, as per instructions of local GP/oncologist