Sudden rising damp on interior walls around a room

this is more likley penetrating damp, caused by failed pointing, a leaky chimney,roof, or an outside drip from a pipe or gutter . that age of property it will be lime mortar in the walls with no cavity or damp course as it is totaly ineffective in that type of building

Rising damp occurs as a result of capillary suction of moisture from the ground into porous masonry building materials such as stone, brick, earth and mortar. The moisture evaporates from either face of the wall (inside or outside), allowing more to be drawn from below. The height to which the moisture will rise is determined by the evaporation rate and the nature of the wall. The normal limit for rising damp ranges from 0.5 to 1.5 metres above ground level. Rising damp may show as a high-tide-like stain on wallpaper and other interior finishes, and, when more severe, as blistering of paint and loss of plaster. Damp walls encourage the growth of moulds, which, with the high humidity, can lead to health problems for occupants. Externally, a damp zone may be evident at the base of walls, with associated fretting and crumbling of the masonry. Having diagnosed the cause of the damp problem, the obvious response is to prevent it recurring by fixing leaks, by removing bridges, or by inserting a new DPC. Good housekeeping measures should be undertaken as well. These will help prevent further damp problems and may reduce the severity of an existing problem to an extent that major works are not necessary. These measures include regular maintenance of plumbing and roof and guttering systems, and attention to site drainage and to underfloor ventilation.

IF THERE WAS NO EVIDENCE OF DAMP PRIOR TO WORK THEN IT IS LIKELY THAT THE WALLS WERE TANKED TO PREVENT INGRESS OF MOISTURE WHICH WAS REMOVED WITH THE PLASTER THIS COULD BE COUPLED TO A FAILED dpc ALSO. yOU SHOULD ALSO CHECK THAT THE CHIMNEY IS CAPPED AND WATER TIGHT AND CHECK THE FLASHINGS AND POINTING IS SOUND.I SUGGEST THAT YOU USE A PRODUCT SUCH AS SOVEREGN HE'YDI TANKING PRIOR TO PLASTERING HOPE THIS HELPS PAUL BBC HOME IMPROVEMENTS

This does not sound like rising damp as im sure it would not appear that quick to that extent, possibly penetrating water ingress( from exterior) as you have checked under floorboards for leaks etc. this would probably be the cause of your sudden damp symptoms. has your plasterer checked externally for anything obvious? rising damp would not occur that fast, its a process that takes months to years to get to the stage you have described. i recommend you arrange for damp proofing specialist or experienced builder to access problem further. this is only my guessing without viewing my self. Hope this helps.(double check for leaks internally).