I just had a couple a few days ago (also called scintillating scotoma) and it's always kind of scary as I'm always afraid my vision will not go back to normal and I will lose part of my vision. They last about 30 minutes. I will sometimes get migraines after or not. It depends.
I started getting them post-op, about 1-2 episodes a year. These persisted despite quitting coffee.
I looked through the literature and although they state it may be triggered by excess estrogen, I'm not convinced because those doses and levels reported in women having ocular migraines were actually quite low and if some women start to get them during pregnancy, it could well be due to other factors other than estrogen like high progesterone levels, the stress of going through pregnancy or several other things going on during this time.
I tend to think it has more to do with stress as when I'm particularly stressed, these will happen. Since my transition, I'm also much more anxious than I used to be so at least, in my case, the culprit seems quite clear. I went to have my eyes checked, did all the possible tests and they found absolutely nothing wrong. They just said I was having ocular migraines, end of story.
Really funny that it should be mentioned the same week I had them! LOL.