I realize this question was posed years ago, but I feel like this information needs to be mentioned in order to prevent this from happening to people in the future.

I am not a doctor, but I had the same thing happen to me several years ago and after doing some research, I've discovered that it's due to beta blocker eye drops. I was given a type of eye drop medication in my exam called Timolol. Timolol is often used to do the glaucoma test as well as to treat those who have glaucoma. Many cases have been found where repeated episodes of syncope (AKA. Fainting) were linked to the patient having an adverse reaction to Timolol eye drops. See this site for more information ---> https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1444852/

The same thing happened to me today, and I have your answer. The doctor put the numbing drops in and then the dilation drops, then left the room, though my mother (a nurse) was still in the room. About a minute later I began to feel sick to my stomach, but blamed it on my nerves and didn't say anything. It began to get worse and my vision started to flash from black to color, and I was dizzy and weak. My mother noticed and asked if I was nauseous, and I answered yes. She quickly found the assistant and they lowered the chair so I was laying down with my feet at a higher level than my head, and the assistant put a cold cloth under my neck. Almost immediately I felt better, and when the doctor came back in to check on me, I asked him why it had happened. He said that it was a common thing, and that the iris expanding pushes on blood vessels in your brain sometimes, and the lack of blood in your brain causes these symptoms. Laying down with your feet up forces the blood back into your brain, so the symptoms go away. I personally believe the numbing drops cause this, as I've had dilation drops without them before and was fine.