Now that the pics have been posted, here are some observations. There is definately a dip to the outside wall, as evidenced by the decreasing gap of the gutters to the shadow board. It also appears that there is some bending of the gutter supports as the gutter gets closer to the drip edge. I am trying to think of anything the roofers may have done that would cause this, but quite honestly, I cannot. Replacing the sheathing and shingles would not have caused this to happen. I see that the dip is directly related to the change of the wall construction where the brick changes to clapboard siding. I suspect there is a change of framing and foundation from the brick section to the corner where the dip is most obvious. I think there could be two possible causes. Is it possible the rafters or rafter tails have settled due to some dry rot where they meet the top plates? If so, I would think it would have been obvious when the old sheathing was removed. If this were the case, the roofers should have brought that to your attention and determined if a fix was possible before the sheathing went back down. The second possibility is that there is settling in the corner of the house. Very hard to see from the pics if this is true. I'd check level on the top of the window and top row of siding to see if the corner has dropped.
With all this said, I think you need a good carpenter to take a close look before blame can be placed on the roofer. I think the roofer did what they do, put the new plywood on and lay the shingles. I would lay odds that there is a problem with the rafter tails. A good carpenter should be able to correct that problem without affecting your new roof.
Without taking a close look, maybe removing the gutter and part of the soffit to see the connection between the roof and wall, and checking the alignment of the rafter tails, it is difficult to diagnose. This may be able to be inspected from the inside if there is access to that area.
