Fortunately, my optometrist didn't see anything wrong with my retina, but she's still concerned. The arcs I see are on the bottom of my eye which indicates that the problem is at the top.... which means that if the retina starts to detach, it could finish detaching quite quickly. I guess that's due to gravity.
I did ask if there was anything else - like an infection, sinus problem, or a tumour - that could cause the flashes I see. She said no, that the type of flashes I'm seeing could only be caused by a particular chemical reaction between the vitreous solution and the retina or something like that. She did say that it's possible that I might end up with a small tear that stabilizes as a permanent floater. I don't know how that's possible - maybe I misunderstood her.
The best news here is that the flashes aren't continuous and I haven't seen as many today as I did yesterday. I'm going in to see my optometrist again tomorrow, and she might send me to an opthamologist. I think that seeing an opthamologist is the right thing to do here so hopefully that'll happen.