Hi, Princessalex. What about coronary artery vasospasm aka Prinzmetal Angina? Arteries can squeeze themselves shut, then that has the same effects as artery blockage. That more or less comes and goes, leaving no sign behind. Did any doc discuss that? It needs 'provocative testing" wherein they inject something that brings it on, but only if you are susceptible.
"a nuclear stress test that was abnormal"
Is that a typo? Because you also say, "My cardiologist swears my heart is fine."
If not that, then "But I can't walk more than 20 steps and my heart starts racing and I'm completely out of energy" might be from something exotic like mitochondrial dysfunction in your muscles. The mitochondria are the powerhouses of the cells. If they're not working right, then the heart might futilely try to compensate by pumping more.
"And to top this off I have now found a lump in my right breast that I'm going to tell my primary doctor tomorrow about. Could this be the cause of all my issues?"
I think that's very doubtful.