Ice damming could be the culprit. Do you have or have you recently had a lot of snow on the roof with a build up of ice on the eave? This will result in water backing up under shingles as far back as the exterior wall and beyond. It will run down onto the top plate of the stud wall and soak down into the wall space into the insulation. Beyond, it will run down onto the ceiling insulation and drywall and soak through, causing a water stain and possibly sagging drywall. You may need to see if you have adequate attic insulation and ventilation to prevent heat from escaping from the interior or venting quickly enough so that it doesn't increase the rate of snow melt faster than the water can evaporate or you will have ice forming at the eave, which is away from the heat escape. Ice and water shield membrane between decking and shingles from roof edge to a few feet behind the exterior wall may also be needed. Good luck in finding help for the proper course of action.