My hemoglobin came back elevated again (17.5) im feeling extremely tired, short of breath, rapid heart beat and abdominal fullness. Why?

?Polycythemia. Many causes: smoking, lung disease, heart disease, medications. I suggest you talk with your doctor. May need to see a hematologist for a further work up. Treatment for polycythemia is removal of blood (phlebotomy). Hope this helps.
Many causes! Dehydration can cause it. But, some other infrequent causes are: advanced lung disease (for example, emphysema), certain tumors, a disorder of the bone marrow known as polycythemia rubra vera, and abuse of the drug erythropoietin (epogen) by athletes for blood doping purposes.