Is LR applying lens distortion, again, by default, perhaps also vignetting correction which would change the exposure especially in the corners?
Most lens profiles that contain information about geometric and vignetting corrections are for raw files only, but a few also have non-raw versions. it is possible that your LR defaults say to apply lens corrections whenever a file is imported and for the lens you're using there are both raw and non-raw lens profiles.
A simple way to tell if this is occurring is to go to the Lens Corrections / Profile tab and see if Enable Profile Corrections is checked or not, and if it is, turn it off and see if the image is more the same as the original raw + your PS patch work.
The lens correction box is not checked when I open my altered photo in the develop module. If I do check the box and enable my lens profile for my camera, the distortion more resembles the original raw image. But now it looks like the photo gets slightly cropped from the original losing a little detail around the right and bottom sides but not the top and left sides. It doesn't seam right that I would have to reapply the lens correction again for every photo that I want to edit in Photoshop.
by the way, my lens profile is a Canon EF-S 18-55mm f3.5-5.6 IS and I'm not sure where to find the setting to see if LR is auto applying the lens correction settings when importing new file
Ok, I think some further investigation is warranted. When does the file look different than the LR version, as soon as you open it in PS or only after you save it back to LR? You should be able to determine this by making an extreme Exposure adjustment or changing things to B&W then open in PS and seeing if the exposure is normal or the color is still there.