My sister is post hysterectomy by 30 years and is now experiencing light vaginal bleeding and cramping.

Grandma's advice is spot on, as they say. Wishing you luck; hope you will update us that all is well.

Tell the doctor ..
When the pain occurs
How bad the pain is
Where the pain occurs. Abdominal? Back? Pelvic?
If there is bloating as well
Does the pain occur at a specific time like after meals, or before a bowel movement.
Any little bit of information you can give the doctor might help figure out what the problem might be.
(diagnosis might be different if she had just uterus removed or if both ovaries were removed as well.)