The initial lumbar spine has a suspicious appearance for a spinal vascular malformation with prominent T2 flow voids, which were proven to be a spinal arteriovenous malformation. These can often be overlooked as a cause for back pain in the setting of degenerative change. This can lead to patients having spinal surgery for degenerative change, which in the end does not help their symptoms.
On the follow-up MRI, there is new T2 cord signal and new blooming artifact, which are both suspicious for a (small) hemorrhage. This would be consistent with the patient's presentation of acute back pain.