Hi welcome to HCM.
Severe low backache
has multiple causes like disc prolapse
, pedicle fracture
or severe muscle spasm.
In recurrent attacks there must be some cause. MRI examination is less likely to come as normal.
If it comes normal then there must be muscular problem. Also ask your doctor to rule out renal stones
because in some cases it may have symptoms like low back ache.
Hope I have answered your query.