Thanks for your query.
your mom is suffering from Hernia.It is in the region of left lower abdomen so most probably it is inguinal or femoral hernia
.Hernia is a condition in which intestine protruded from natural opening of body when intra abdominal pressure increases & it goes back when it decreases that is the reason she gets swelling on standing & invisible on sitting or sleeping.
there are possibility of incisional hernia
if it protruded from operative scar.
The complication of it is strangulation when it comes out but by any reason not returned inside this is an emmergency.
Just keep hand on the part while you have strain like passing stool ,laughing ,lifting heavy weight etc.the perment solution is surgery.Thanks