Check for faces inside the mesh. These are referred to as interior faces and they mess things up in ways similar to what's going on in your model.
One way to find them is:
- Go to Edit Mode (Tab)
- Make sure to deselect everything (A key)
- Open the Select menu (bottom of the 3D Viewport window)
- Choose Select All by Trait > Interior Faces
If anything ends up selected, hit X or Delete.
If that doesn't work, you'll have to hunt it down by hand. To do that:
- Enter Edit Mode
- Turn off your Subdiv modifier (eye icon in the modifier pane),
- Delete a face near the problem area,
- Rotate your view around and peek inside.
If you see an interior face, select and delete. And when you're done, of course, replace the face you deleted.