Hello from JustAnswer.
It is certainly true that the low haemoglobin can cause these symptoms, because the red blood cells carry oxygen throughout the body, including the brain, and when there is insufficient oxygen, then every organ in the body may not work properly.
There are many possible causes of a low haemoglobin or anaemia. It is true that one of the most common causes of anaemia in young adult women is iron deficiency from blood loss during periods. However, you are correct that if you are not having periods since the haemoglobin was 12.8 in Dec 2014, it cannot be blamed on the periods.
It is possible that it could still be iron deficiency related to other sites of blood loss, which if not readily apparent, is usually in the gut such as from ulcers, vascular abnormalities, inflammatory bowel disease, or growths. There are other nutrient deficiencies that can cause anaemia, such as vitamin B12 or folic acid deficiency. It is also possible to get anaemia because of increased breakdown of red blood cells, such as from haemolysis.
The cause of the anaemia cannot be identified from the haemoglobin level alone. There are many other tests that would need to be done to determine the cause of the anaemia. Your doctor should have already started performing additional testing once it was learned that the haemoglobin was 7.5.
If I can provide any additional information, please let me know.