What Causes Dizziness, Lightheadedness And Weakness?

im a 29 yo female. i have been getting very dizzy,lightheaded and weak w when i stand.not only that but i also get these spells when i walk not even 15 feet where my heart starts to race and pounding so hard it feels like its gonna come out of my chest. i get headaches and sometimes my hearing gets muffled during these episodes. my doc. has ruled out my blood pressure, anemia,diabetes,and my thyroid. still have no answers.