The ER doctor is right. Tiny atelectatic bands are very small areas of the lung that are not fully expanded so they are getting less air. They are nothing to worry about because they are so small. Your sharp back pain, especially if it was worse with breathing, could cause this problem. Also small scars from a past infection may look like atelectatic bands.
The most important thing was the determination that you did not have another pulmonary emboli (
PEPes planus). In truth, your description of the pain is more suggestive of a musculoskeletal problem.