I actually talked to a nurse about implantation dates today and was told that 5 is technically possible. It seems it's just extremely uncommon and so it's not counted in the typical possible range. The egg loses its casing on day 5, apparently, so implantation is not outside the realm of possibility.
That was news to me, but there you have it!
Dizziness was definitely one of my symptoms this time, but not until 8dpo. I was having some dizziness last night, and again today, which was a contributing factor to my early test (the big one was nausea without feeling ill).