We had exactly the same problem a few months ago with a GST fusion protein
in pGEX-2T. Our 18kD protein ran consistently at 32kD on SDS PAGE gels.
Unfortunately I can't give you any explanation for this as we never found
one! We verified our protein by high-res mass spec and the protein was fine.
Since you are having the same problem with a different protein it looks
like something to do with the GST. Any explanation of this phenomenon would
be welcome - please let me know if you find anything out.