What your describing sounds like a symptom of an ocular migraine. Basically I see it when I'm in dim or dark light. It looks like a spot right in front of my vision that kind of has a wiggly/wavy movement on its edges. I also see this when I wake up in the mornings, sometimes.
You should have your eyes checked yearly so if you already had them checked this year and nothing was found, especially if they were checked when you were having this symptom, there is nearly zero chance this is a detached retina.
Though retina detachment can happen to anyone, its way more likely to occur to older people, people that are extremely nearsighted like unfortunately i am or because of an accident or blunt force to the head or face (bungee jumping has caused many retinal detachments in young people). I take it you haven't jumped off a cliff lately...:)
I see two types of flashes in my vision. The most common one is when I go from bright light to dim light. This causes a constant flickering in my vision, mostly around the peripheral, which is very annoying by the way. Sometimes when I am having a silent migraine or I am especially anxious, this flickering gets constant. It truly is extremely uncomfortable. Once my eyes adjust to the darkness, it seems to go away. In the morning, the opposite is true. As my eyes try to adjust to the light, the same flickering/flashing effect occurs.
The second type of flash I've gotten, is rarer and has only happened to me a handful of times in my life. Its like a lightning flash, where my whole vision goes white for maybe a second, if even that. I've heard other people experience this too and its not the eyes, its the brain. It seems to be related to the same migraine issue.