Definitely a structural engineer would be the ay to go, but make sure its also a good one
They aren't cheap, but since they aren't working for a builder, or associated with one for the work, they should be able to provide you with an honest report on what is probably the issue. They can also provide a report and repair details of what may need to be done to fix the issue by someone and can be used to obtain any required permits.
Well those are the obvious first questions to ask, I just figured i would give him examples of the others......