You have chronic back issues. It was obvious when you were talking about the awesome pain in your back after deadlifting a few months ago.
Keep doing what you are doing and work on form and eliminating any lumbar flexion (even if it costs you depth, everyone will squat different depending on anatomy and hip flexibility), but once your discs are damaged in such a manner they never heal as such, they only settle, and your back may not be suited to high compressive loads you get from heavy squats and deads.
If you find yourself in a cycle of causing yourself back pain by squatting and deadlifting, do another exercise, or one day the disc will be too torn up and the pain may never leave you and you will not be a happy chappy then. It's up to you to determine the difference between the pain of effort and the pain of tissue damage. A burning or stabbing pain in your back is not the pain of effort.
I'm cautioning you because most bros will be all, harden the f**k up, lift heavy dude and have awesome definition on my back etc etc, but they've really got nfi about injury or life in general.
Have a look at this. This is Rippietoe squatting.
http://www.youtube.com/watch?v=FVKEl4Wxoqc
Notice how at the bottom of each squat he reaches his hips end range of motion, then gets a little more by flexing his lumbar spine. Terrible form, and that is how you increase the load on your lumbar discs by a large amount (specifically anterior shear). So don't think you will necessarily get good instruction on squatting.
If you have good healthy discs (and most people squatting do), you can get away with it, and it will probably work your hammies as well when you flex that lower back. But it is at the cost of very high spine loads. If you start doing that sort of thing with existing back problems and heavy weights you will hurt yourself.
This post might cause a minor shit storm but everything I say there is true, the responses will be along the lines of 'I lift with some minor flexion and I don't hurt myself so it must be the same for you', or 'you've just got to have a better training attitude' or 'my back hurt for a bit then I got a massage and it went away now I squat heavy' etc etc. It just means they're not injured and unfamiliar with the mechanism of disc injury and back pain.
Good luck and train smart.