You are a strong dude, but there is absolutely no way you can feel stable squatting in tennis shoes -- especially on those ribbed stall mats!! You really need a flat surface to push from. In the later reps your can see your heels get 'light' as you push your toes push awkwardly into the squish of the shoe, into the squish of the shoe into the ribbed stall mat. As far as the pain goes, if it really is just a 'tweak', then lower the weight to where it doesn't hurt and build back up, tweaks won't last forever.