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There are multiple options regarding pano stitching (assuming photoshop). It would be helpful if you detailed your workflow and which sequence / option you chose.
File>Automate>Photomerge>Reposition is my usual choice for working pano ... usually from uncorrected files for first attempt. Having made distortion corrections in on file can force a need to "bend" another file more than might have otherwise been necessary. Your left & right seem pretty good ... it looks like the center is "bending" to meet them more than would have been caused by lens distortion.
How many images were used and how much "overlap" did you have for each image when you capture them? Also, were you shooting in horizontal/landscape orientation or vertical/portrait orientation? While it takes more images to get the coverage, I typically shoot pano's in portrait orientation with about 1/3 overlap.
There are multiple threads (search "pano") in the Alternative Gear Forum for Pano's if you're interested in getting more insight to pano's with "Pano-mania" being the more prolific of the threads over there.
WARNING: View at your own peril. Side effects can include, increased image file sizes, leveling gear acquisition, nodal point discussion, drooling over gigapan images and potentially initial feelings of inadequacy. Other side effects can include a new way of seeing things, a liberation from standard cropping & framing and a sense of drive and accomplishment. Additional side effects may vary that are individual in nature. Consult your FM friends for additional help should you experience any of these. While there is no known cure for "Pano-mania", a reasonably normal life can be achieved with occasional use of a square format image. Pano-mania is not commonly found in regular people, but exists primarily in individuals that are not satisfied by status quo. Pano-mania is a growing sympton, that has recently been proliferated by advances in CPU speed, more RAM and software improvement. While Pano-mania is not for everyone, a fulfilling life remains possible, even as Pano-mania varies in degree with each individual.