Did your form crash and you ended up with some asymmetrical pulling?
Do you/ do you see a chiro?
Have you tried any kind of massage and or trigger point work?
What have you, or are you doing to rehab it?
I don't necessarily think a belt will stop a strain and in fact gives some guys a false sense of security to protect from an injury. I never or rarely use one for deads and believe you can never have a strong enough lower back or core. Limit squats, sometimes I do.
I have hurt or strained my upper and lower back quite a few times over the years doing bending exercises.