To me that sounds like a visual migraine. I get them ocassionally, usually triggered by stress and sleep deprivation (which would be expected after having a baby). It's like a migraine with the visual aura, but without the headache. For me, it has nothing to do with pregnancy and labour, but I can see how the postnatal period could trigger it. For me, it's a flashing zig zaggy line that curves around my field of vision in both eyes (if you close one eye at a time, you'll still see it in the open eye).
That said, I would talk to your midwife about it urgently as visual disturbances can be a sign of other more serious conditions, like high blood pressure or stroke. But if it is a visual migraine, it's very common and not serious and nothing you need to do other than try to rest and take care of yourself.