Weird Knee problem, suggestions?

Fibretip

For the last couple of weeks i've been noticing this weird feeling in the back of my knee. I'll be sitting, standing, walking, anything, and all of a sudden i get this really hot sensation behind my knee, like someone has just put a hot water bottle there. There's no pain at all, just this sudden hot sensation.
I dont know if i should be worried by it or not? My wife suggested it might be circulation related, so that gave me food for thought. I've got a doc app next week anyway so i'll bring it up then, i just thought i'd throw it out here and see if anyone had any thoughts. If it was circulation i would have thought i'd have some kind of discomfort, tingling, pain or whatever.

Things i considered... i started driving a month ago, and am out for about an hour most days. I've just had a baby so i'm on my feet a lot swaying from side to side, bouncing etc... those are the onyl things that have changed, i wonder if i've strained it or something.

tl:dr - i keep getting a weird hot sensation behind my knee... any thoughts?

thanks muchly!