How long does the flash last? Do you see black things floating around? Do you get a headache with it? How is your vision? Do you have any other medical conditions? What is your age?
This could be one of three things.
One is your low blood pressure. Sometimes, people will see flashing lights and lose part of their vision. Some people see tunnel vision when the blood pressure drops. Although if your blood pressure is stable (even if it is low), then this is probably not likely the cause.
A second condition that can have similar symptoms is a migraine episode. You can have the visual symptoms (flashes, losing part of vision) that may or may not be followed by headaches. Usually the visual part (called the aura) only lasts about 15-20 minutes though.
Most likely, I think that you may be experiencing a posterior vitreous detachment. There is a gel inside the eye. If you are very near-sighted (or with age or with trauma), the gel breaks apart into little pieces and float around. During this process, the gel can pull on the retina and cause you to see flashes of light. This process is harmless but annoying. Eventually, the flashing will subside and over time (many months) the floaters will sink down out of the line of vision (so you won't see them constantly, but only at times). However, these are the same symptoms as a retinal detachment (which is a major problem). When someone has these symptoms, I always have them come in to make sure the retina is OK.
There is no urgency, but you should see your eye doctor just to make sure the retina is OK.
Hope this helps and good luck.