I've 'tweaked' my back several times over the years, and most times it was just from making a wrong movement, and not from lifting something. The worst time was in about 1994 while moving wrong....while lifting something heavy. I was holding a very heavy (about 18#) pinch bar with one hand and holding onto a ladder with the other hand, and reaching about 3 feet to my left. I felt a little 'tweak' but nothing serious....so I thought.
About 4 hours later I was in severe pain.....to the point of tears.....it hurt when I moved, and it hurt when I didn't move. I took OTC pain relievers like they were Jelly Bellys.....they didn't help one single iota. I went to a chiropractor the next morning....that didn't help. I went to my folks' doctor, who is also an osteopath, on the way home from the bone-cracker. He took x-rays and found that I had 5 vertebrae out of place....each pretty much going different directions, and pinching a couple of nerves.
I got a prescription for muscle relaxants with painkillers, and a shot of cortisone, along with the Doc manipulating my back and working at getting things back in place. I went to the Doc's office 5 days a week for 4 weeks for treatment. I lived in my tiltable, high-back office chair for those 4 weeks....it was the only place I could even begin to get comfortable. I only slept about 5 hours the first week.....and watched a vast quantity of late, late shows. After about 4 weeks I was finally able to get in and out of bed with great difficulty, but could only sleep a few hours at a time. I wore a back brace almost constantly for about 3 months.
It took over 6 months to get back to near normal. I haven't had any major episodes since then, but I'm always pretty conscious & cautious about stretching, twisting, and lifting.....especially at the same time!
BTW, here's what an 18# pinch bar looks like....it's approx. 5 feet long: