Who would I call to fix this...?

[secuono]

Well, ever since we moved in, we've been noticing some odd wood darkening when it rains heavily. The attic is clean and dry, there are two additions to the house. Last addition's roof does have one small hole and leaks, but we can't seem to find it...
But I digress, there is a wall before the bathroom, which is one of the additions. On either side of this wall, the floor gets dark and looks to have started buckling/warping.
I'll get pics!
All I can think of is somehow, where the original 2 story back wall touches the new addition roof, water is getting in. But the walls aren't wet. Though, we did find and replace one section of wet/crumby drywall. It was right over one of the dark areas, it seems like the moisture wicked up into the wall. The 2x4s looked dry and fine back there.

My question is...who the heck do we call to get this figured out and fixed? A roofer, a house builder? I just have no idea....

Alright, pics.
1st, the original exterior of the house, clapboard and original paint. No damage to be seen.
2nd, directly behind the wall/door of the 1st picture. You can see the dark wood, it rained 2 days ago.
3rd, The, I believe, non-supporting wall. Bathroom is behind there. Both corners have wet wood. New paint on walls.
4th, has dried up, yellow marks where the dark area would be.
5th, Hall between livingroom and bathroom. Wet area and wall we replaced in yellow. Green ring is where the exterior wall is/was. You can see the layers of linoleum, plywoods and tile over the top of the original add on bathroom floor.

[GothicHome]

The original roof above the addition, side or gable end. If side I could see water getting under the shingles or behind the facia if the gutters flood or do not drain. The water wood flow on to sofit and then over the top plates and then follow the studding till it reached the header of the door and follow the door frame onto the floor. All of this on the inside of the old exterior wall. Who would you call? If not up to the task I think a general contractor, a good contractor should have the knowledge and experience to trace the problem from the source to the end.