Thanks for the info. I really tighten my core (push against an invisible belt/wall), but when I hit about 265 on squats I went down on the first rep after warmups and felt a weird pain on the left, near my diaphram. Searching webMD was inconclusive, so I took a week off and when I restarted I was more careful/cautious about form, breathing and how hard/where I pressed/squeezed in my core.
When I got up around 285 and started running out of plates, I decided to video myself and on review decided form was okay (bar was in the right location and moving smoothly up/down), but I wasn't going down deep enough. So I dropped about 50 lbs from the bar and started working deeper squats, which after the initial adjustment felt fine.
Then came fall when we're stupid busy, fell off my regimen and restarting Monday sucked. 1 plate/side felt like nothing, 2 plates/side was tough but okay on the final warmup set, but 100lbs/side just didn't feel right in my knees - like I was pushing too much at the knee and not at the hip. On top of that, my left rotator cuff hurt holding the bar behind my neck because it wasn't stretched out good (it also often hurts on OHP). I ended up dropping to 185 to finish out the 5 sets and was generally unhappy with the workout. I think I only did 205 on DL.
Sorry for running on - just frustrated with the regression and always worried about starting back, not realizing how stiff/weak I've gotten, and worried about injury. The belt made me think about gear other than weights & rack.