Sometimes I Get Lightheaded, Feel Out Of Touch And Like

Sometimes I get lightheaded, feel out of touch and like I may pass out (but don't). Sometimes my hands and arms get weak and shake (almost like a seizure) for up to 15 seconds before I feel normal again. This happens at least 4 times each week and tests to date show my heart and brain are normal. Any ideas what might be going on?
Thank you, krista