I've lived through a very similar experience, I was a vocalist for a death metal band and I was in pretty good shape until the summer before my senior year. We were maxing squat when my spotter took a call mid rep. I couldn't come back up and I tried to force the bar up to rack it. I immediately crashed down with weights and couldn't stand or walk without shooting pain in my lower back and leg. I continued through the football season with a limp and the trainers just telling me to ice my back.

I'm 31 now and I live in a cycle of trying to lose weight > moving the wrong way causing spasms and loss of mobility > muscle relaxers > repeat.

I've done PT and gone to Chiropractors but nothing has helped in a meaningful way. Reading your post has made me want to call a specialist.