Mri of lumbar spine says i have a "left laminectomy defect, status post discetomy." but... i never had back surgery - ever. should i be worried?

No: Not worried, no. It could have simply been a misread by the radiologist; sometimes the findings of surgery can be subtle and so a radiologist may have misinterpreted the study. But the first thing I would actually check is that the MRI that is being reported is actually yours. Sometimes results can be attributed to the wrong patient, particularly if someone has the same or similar name.