Squats ATG: why does my knee hurt?

[CaliforniaKing]

So I just recently switched to ATG squats and the first week I did ATG, I did a lot better than I thought. I felt like the form was good, I lifted more than I expected, and my knee didn't hurt at all the next morning.

Today was my 2nd week, and last night I was basically trying to perfect my form. I found a bar in my house and practiced a little bit with it. The next morning (today), I had a little pain in my left knee and can't figure out what I did wrong. While I was doing the squats, it felt like the form was good. I even did a couple in front of a mirror and the form looked decent.

What do you guys think I may be doing wrong? One thing I did notice was that on a couple of my squats, my lower spine felt like it was cracking, but I figured out that my stance was too wide and it went away.

(BTW, I use high bar if that matters)

[CaliforniaKing]

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