I was having light flashes on my left eye , now i see a floating black dot what is happening?

Flashes of light: Sometimes when the vitreous (gel in the back of the eye) detaches, it can cause flashes of light and floaters. However, so can a tear in the retina. I would definitely recommend a dilated exam immediately by an ophthalmologist.

Vitreous Detachment: You are describing a vitreous detachment with floater; usually benign. A retinal exam is needed to fully evaluate. Rarely this can also cause a retinal tear. If the vision becomes distorted, partially missing or you see thousands of specks in your vision this is a retinal detachment; an ophthalmic emergency.