Vertical heterophoria would not give you light flashes. Light flashes usually result from the vitreous pulling on the retina as the vitreous goes through its natural process of separating from the retina. Most of the time, this process causes no damage, but in 10% of cases, a retinal tear can result. Your ophthalmologist needs to perform a gonioscopy to be sure that the anterior chamber angles are open and not narrow; very narrow angles could cause a narrow or angle closure glaucoma attack with eye pain and pressure. After that you need a dilated retinal exam with scleral depression by an ophthalmologist, perhaps a retinal specialist to be sure that there is no retinal tear.