I wasn't really sure what to call it, but the closest thing I can describe it as is faintness. Also this is going to be a long post, with as much information as I can possibly put.
So this doesn't happen as regularly as it did to me a few years ago. Occasionally, I start to feel extremely faint, and would collapse soon after, but I would still be conscious. It was a weird form of consciousness, where I felt like I wasn't in my body (closest thing I ever experienced to it was being extremely drunk to the point where i was puking). I guess depersonalisation is the accurate term?
My vision would get messed up as well, sometimes i'd experience double vision, others all I could see was purple, and what seemed like thousands of pixels. Following this my vision would be blurred until I felt normal again, normally 10-30 minutes after the faintness starts.
I can trigger this to happen through strenuous exercise, but this 'faintness' can happen to me at anytime. First time I experienced it was in 2011, and I was just sitting down doing schoolwork.
Pain also seems to trigger it, I was knocked over while playing football in 2015 and it happened after what felt like a really bad cramp in the leg. Following this my parents booked a GP appointment, they thought it was anxiety (funnily enough i also suffer from that) and just went with a blood pressure test and urine test, which didn't turn anything up. I was an anxious 14 year old at the time, so I didn't ask for further tests and neither did my parents. It never even crossed my mind to request a blood test, something I never did until this year.
Last time I experienced a bad episode of this was in January 2016 after pushing myself too far in the gym. Since then, they've become less frequent, despite the gradual decline of my overall health. I've had more minor versions of it, just sudden faintness, no blurred vision, no depersonalisation, no collapsing.
As you can imagine I've done a lot of research on what these episodes could be, and google tells me anything from panic attacks to heat stroke to heart failure because of how universal the symptoms are. I've had blood pressure tests done, one in 2015, one in 2017, both came back normal, so even low blood pressure seems unlikely to me.
I've never asked a forum about it though so I'd thought I'd finally get some closure here, so I can bring it up with a GP next week when they contact me about my most recent blood test.