What could cause temporary hearing loss in my left ear and darkened vision during exercise (high intensity only)? it goes away immediately with rest.

Visual obscurations: Your description of visual losses with heavy physical exertion is consistent with what happens in cases of what used to be referred to as pseudotumor cerebri now goes by the name of (idiopathic intracranial hypertension). Hearing can also be affected if pressure inside the head suddenly goes up. Rest typically relieves the condition. You should visit a good neurologist for a diagnosis.