More than likely, if this has been going on for a while, you have a mold issue.
You can break a piece of wall off, just as any professional that comes out will do. Then you will know what is behind the wallboard.
Just remember to open a window for fresh air when you do this..it can get overwhelming.
If it is a really bad issue, call your insurance company, they may take care of it.
Don't pay $$ to have someone come sample the air, unless you find nothing behind the wallboard.
And make sure you fix the water problem , or it will come right back.