This was happened because your NT login created with default schema with your login other than dbo.
Try to varify what's in default_schema_name:
SELECT * FROM
where name like '%<your login>%'
See what the default_schema_name is.
I believe the default_schema_name is your login.
Then ask your administrator to change your default schema in the database that you are trying to use, as you do not have permission to change the default schema.
He might have to use the following command.
USE <your database>
ALTER USER [<your nt login>] WITH DEFAULT_SCHEMA = dbo