For many years I had this problem, mine would sometimes occur twice a week or more often. I only had the visual symptoms, never any headaches. The structure that you see is a scintillating scotoma, which means colorful blindspot. The eyes are wired in a way that most people are unaware of. One half of EACH eye is connected to one brain half, the other half connected to the other brain half.
An Ocular Migraine occurs when the blood vessels spasm, reducing circulation to the Visual Cortex of the brain. This produces the Scintillating Scotoma. It should be in one half of each eye, so it may be in the right half of the right and left eyes, or the left half of the right and left eyes. Some people perceive it to the right or left, but if you look close, you'll see it in both eyes. The following picture is rather like what I saw during a phase of each migraine.
You have been examined by a doctor, which is essential to rule-out any serious causes, but the phenomenon itself is common in migraine. I had mine frequently for 11 or so years, up to several times a week, each episode lasting about 45 minutes, with an occasional doubling for 90 minutes. There's a book by Doctor Oliver Sacks about the many forms of migraines, producing, if memory serves me correctly, mosaic migraines, Lilliputian migraines, and more.