Cakewalks encoder is the LAME encoder as well. Ive never had any issues with it.
Also there is no loss going from MP3 to MP3. once the file is converted, there is no further coding needed.
This is of course as long as the same setting/bit rate, exc is used.
My guess is there was some sort of routing issue, or a fader dropped somewhere, either the master bus fader (which should be left at 0db) or a track fader not at 0db.
OR...
You had a different bit rate on on your export.