Probably more helpful to see the set where it actually happened, but from what I can see here:
1. You let your back round a little at the bottom
2. Then look up as you ascend and try to lift your chest/do a back extension instead of drive your hips
While it's unusual for these issues to manifest in a tweak with 180 lbs, some people are just unlucky. You need to learn how to keep your back flat/extended and drive with your hips. These things are covered in great detail in the book and many articles and videos on this site. If you've already read and watched those all multiple times and still can't do it, I will try to help further, but sometimes some in person coaching is needed for people who really struggle with those things.
Were these classes taught by an SSC? Recently?
You're not feeling a decent amount of stiffness and pain?Naturally, I stopped and now I'm not feeling a decent amount of stiffness and pain.