Blood sugar and Hypoglycemia
Immune system dysfunction.
Fighting an Infection.
Malnutrition.
Severe stress (stress that changes the immune response) ...
were some of a main list.
Others:- Malnutrition, Thyroid, Hormone, Iron Deficiency, Dehydration, Lupus, Poor Blood Circulation / Clotting and Other Causes also came up elsewhere.
I don't want to worry you, but some of the things with the symptoms you prevously mentioned, need urgent medical investigation.
Mark

Added: you can even be eating plenty, but still have Malnutrition.
Eating plenty and Loss of weight (going thin) can indicate the body is drawing on its reserves, to try and get vital nutrients.