It sounds like there may be a leak in the metal flashing that surrounds the chimney that is allowing water to dampen the framing. Another possibility might be a damaged chimney flu. Creosote and soot could be wet from rain and carried through cracks in the flu and then through cracks in the brick or mortar of the chimney and down your bedroom wall.

We suggest that you have a chimney sweep inspect and clean the fireplace and flu and make any repairs that may be needed. If this doesn't solve the problem, have the flashing that surrounds the chimney at the roof water tested for leaks.