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How long did it last for - 5 to 15min? It sounds like an ophthalmic migraine aura. People can get visual auras that last 5-15min randomly and without a migraine. No need to worry. There's no prediction as to how often it will occur or when; but now you know what it was. If you ever experience intermittent flashes of light more than a constant shape then see an eye Doctor for a dilation to evaluate the retinal tissue for holes/tears. But the way you are describing it sounds like a typical aura.
Any other questions I can help you with? =)
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