For a few minutes i was seeing green spots. they are gone but i have never had this before. what causes that?

Ocular migraines!: Ocular migraines are a form of migraines that take place in the occipital cortex (part of the brain that interprets/controls vision). They lack the severe headache as common migraines. The symptoms are usually all or mostly visual and are typically described as flashing of light or ziz-zag lights in the vision. They usually last less than 30 minutes and affect the vision out of both eyes

Afterimage.: The most likely cause of the spots you saw were an afterimage that occurred after looking at a brightly colored object.