I am wondering if craving salt, feeling nauseous, & having difficulty concentrating can be due to extremely low iron. ?

Craving of salt: usually means your serum sodium level is low. Hyponatremia (low sodium) can cause cognitive impairment, confusion and even seizures if low enough. I'd have this checked right away, along with your iron, CBC and ferritin. Low iron will cause restless legs, fatigue and result in problems with delivery of Oxygen to the tissues if low enough.

Possibly: If your iron level is so low enough to cause you to be anemic, then you could have similar symptoms to that. I would recommend having a blood count performed, to see if you do have anemia and low iron. If not, then your doctor can look for other causes of those symptoms at the same time. Good luck.

Blood loss: Red blood cells use iron to help carry O2 to the tissues. Low iron can occur w blood loss, diet, poor absorption of iron, other.If enough blood has been lost, salt craving could occur. Low iron can cause "pica" (craving non-food items or ice), but salt craving is more likely due to low blood volume.Difficulty concentrating is possible, nausea raises question of gastritis, vs side effect iron tabs.