The long-term use of Nexium, as well as other meds intended to decrease stomach acid, can definitely be contributing to your problem. I suggest getting a full vit/min panel run.
Reflux meds can inhiibit the absorbtion of iron and B-vits. Calcium absorbtion can also be inhibited by reflux meds. Its really important to get tested and find out what your levels are before supplementing. If you have too high a level of some nutrients, it can cause more problems than having low levels.
Please keep us posted.