It helps to put the flashing lights into context.

If it happens when you get up from sitting, it can be related to a blood pressure/flow changes.

There are some dangerous issues to be considered by they are not as common as the benign causes.

More information about these flashes would help.

If these flashes have been frequent since you recent head injury, you may want to get your eyes check by an ophthalmologist. It can be worthwhile to reduce anxiety loads even if your eyes are fine.