Damp proofing already treated room?

The problem you probably have is that your internal walls were not treated as well as the external - only half the job done. I owned an old house that had a similar problem but the treatment included injecting the internal walls too, removing the plaster half way up the wall and re-plastering with a damp-proof render before being re-plastered. If that is not done then your internal walls will continue to suck the damp in.