Dehydration = sugars all over the place?

Bit worried today, having had readings pre and post meal of no higher than 7 for 6 months I was surprised to find a reading if 12.6 for no obvious reason after lunch today. I ate nothing I've not had before and had normal readings. I also did not feel 'high' which I have started to be able to feel now.
What I did wonder is if it was dehydration, after it not coming down much over 30 mins I drank 2 pints of water. 40 mins later its 6.5.

Here's my real question, is this reading a false high? My sugars are not actually high but my blood is more concentrated giving the appearance of it being high. Is this as 'dangerous' as a high from genuinely too much blood sugar?
I will of course in this hot weather ensure I drink more but just wondered if anyone had any ideas or had discussed it with their doctor?

Thanks