Thank you for the query.
The most probable reason of your symptoms is Crohn`s disease (as all other reasons has been ruled out). This is the inflammatory disease which can affect every part of digestive tract. It causes all mentioned by you symptoms including joints inflammations. If this diseases affects small intestine
only, to diagnose it CT with contrast (so called enteroclysis) should be performed (you have not mentioned such test). It is strongly advisable to have it diagnosed as there is biologic treatment with antibodies
usage available. Such treatment gives many positive responses.
Hope this will help.