Title was : huh!?
I changed it in the hope that somebody sees it and replies to your question
I was falling asleep after a WBTB, and all of the sudden I saw a really really bright light - as if someone just turned on the lights in my room and amplified the light by 5. The minute I noticed it it went away, but it was very very strange (I haven’t quite heard of this before here). Was this just weird phosphene manifestation?
It depends, where you holding a flashlight up to your eyes? Then no.
Were you just waiting to fall asleep? Then yes, most likely.
Tibetan monks say there are 3 flashes when you fall asleep (and stay perfectly conscious during the process, which is rare): a white one, an orange-coloured one and… I forgot the third! 
It was perhaps something like that…
The third flash is a hot girl.
Lol. Should be a red flash then? 
Anyway such flashes happen from time to time, might be some form of HI and/or phosphenes most likely. At times when I do WBTB + WILD and fall asleep consciously, I see a kind of flashing white light for a few seconds to perhaps one minute. Could be caused by the visual cortex responsible for your eyesight shutting down.