Knee Pain

Not knee pain but I have developed a horrible, sharp stabbing pain right at the back underside of my left heel. It only happens when I bend over or crouch (ie when tying laces), but it is fierce. It is like my heel is being ripped in two.

I have recently changed the handlebars on my fixed gear bike which has had a very negligible change in riding position. Up until now I have been running a bullhorn with a slight drop, and swapped for ergo bars with aero levers. There is no other discomfort to speak of.

Any insight into whether the two could be related would be appreciated.

At a very large guess, it sounds a bit like Plantar Fasciitis.
There are loads of stretches on the web which will help.
You may also want to change your saddle position.