ADO.NET - no database change

Hi. I'm using Visual Studio 2010 professional and .Net framework 4.0, and I have this weird problem with ADO.NET.

I created an OleDB database for test purposes. A dataset was automatically generated by VS, and I used it to delete/update/insert data.
For example, here's a code I wrote to remove all the records from a table named "Departments":

Code:

           CoursesDBTableAdapters.DepartmentsTableAdapter da = new CoursesDBTableAdapters.DepartmentsTableAdapter();
            CoursesDB.DepartmentsDataTable dt = new CoursesDB.DepartmentsDataTable();
            da.Fill(dt);
            foreach(DataRow row in dt.Select())
                row.Delete();
            da.Update(dt);

It works fine. The records are indeed getting deleted. But after like a minute, when I run the program once again, the original data (the records that have been deleted) finds it's way back into the database.
Any ideas what might cause that?