The difference between a memory and a flashback is the tense. A memory is past: you know it happened then. A flashback is present — the body reacts as though the event is occurring now, with everything that implies physiologically.
They are not necessarily visual. Many flashbacks are somatic — a physical sensation with no image, a tightening, a pain that returns without cause. Others are emotional: a wave of dread or shame, disproportionate to the context, with no image attached. The latter are the most frequently missed, because they don't look like what people expect a flashback to be.
The trigger is usually sensory and mundane: a smell, a tone of voice, a quality of light. It has no connection to the content of the memory, which is what makes the reaction seem to come from nowhere.