This is Dr Basu, Experienced Specialist.
I am here to address your concerns and provide great service.
So far you had all the needed tests done to rule out any GI bleeding or cancer.
You also had CT scans of the lungs.
However drop in oxygen saturation can be caused by the blood clot in the lung blood vessels as well known as pulmonary embolism
This is only detected by the CT angiogram of the lung blood vessels.
Yes low ferritin level suggests iron deficiency --given the symptoms likely moderate iron deficiency.
This can also cause fatigue
and drop in oxygenation with physical exertion.
Ferrous sulfate 325 mg tablet contains about 65 mg elemental iron.
You need about 2 such tablets daily with a glass of orange juice or vitamin C for better absorption.
After 2 months of iron supplementation you should see normalization of your ferritin and hemoglobin levels.
Sometimes, you may need iron trnasfusion if you do not respond well to oral iron supplementation.
But that is a later consideration for now.
It was my pleasure to help you today.
If I have answered all your questions, Please click on Excellent service so that the website allows me to assist you further :)