Why do i get dizzy when i lay on my left side?

Positional Verigo: You might have benign positional vertigo. See an ent. If you do, this can be easily treated with some maneuvers.

Inner ear: Could consider local inner ear problem, such as "otolithic dysfnctn", where the receptors are irritable and shift when on left, but could also wonder if posture affects the vertebral artery in the neck, with some minimal compression. These issues can be sorted out with vestibular testing and even a ct-angio, .