I think it is normal for this to happen. The pain will often come back, maybe even in a more severe way, just when you think you're recovering. Just to make you work a little harder to keep it away. The method of going about keeping it away though, I'm struggling with. I'm sure your form was fine. The brain knows you may have been concerned about your form, and took that opportunity to give you pain in your lower back, a spot that you were already mentally sensitive to. Don't let the brain get away with scaring you out of doing something that you are totally capable of.
Wish I had better advice for you, hoping for more comments. I was going to start a similar thread today, as I was finally starting to recover and then I woke up two days ago with really bad neck and back pain again, out of nowhere. It's so very obviously TMS but I can't seem to shake it. It's like I can hear the brain taunting me and I don't know how to gain control.