The trauma could have caused some amount of posterior vitreous detachment
(PVD). In this condition the thick gel in the middle of the eye gets detached from its surrounding retina
giving rise to symptoms of flashes of light and floaters
. It is good to know that your eye has already been examined by an ophthalmologist . It is important to get this exam done so as to check for the intactness of the retina. Since your eye did not reveal any retinal problem, I guess you can stop worrying about the flashes of light and floaters as it is only due to PVD ( which is otherwise not a serious issue). At any point of time if the flashes of light increase or there is a sudden shower of fresh new floaters, along with blurring of vision , then do get the retina examined immediately by a retina specialist so as to look for retinal holes or tears or detachment.