Left side of my foot -- 8 inches up leg numb? First of all -- I know this isn't Medical-Overflow-Exchange, sorry.

But, I think my issue may be cycle related. I've been riding a lot lately, and actually just started riding Fixed again. And for about 3 weeks the whole left side of my foot and about 6-8 inches up my leg are completely numb!

I've read it could be related to my back, I've been cycling and when even not cycling carrying a sling bag for work; with a Laptop and a few notebooks in it.

Anybody come across this before?
I've heard that sling bags are terrible for your back. They don't distribute the weight evenly at all, and can cause major problems I don't know why they are so popular. I guess backpacks look too "high-school".  But I would start by getting a backpack. They make a few of them that are specially equipped for carrying laptops.  

If it's the bike, you might want to check that everything is aligned properly.  Ensure that your seat is straight and level.  Ensure that you handle bars are properly aligned. Check for a bent axles in the wheels and bottom brackets, although that may be less likely.  

I guess a couple clarifications could help diagnose some other posibilities.  Do you have brakes on your fixie? If you do have brakes, do you use them? Do you wear bike shoes or have toe clips?  Is the pain always there, or only after a prolonged ride? If it's continuous, I'd definitely get myself to a doctor. That's way too long to be numb in such a large part of your body.