Code :
 private static void Window_MouseMove(object sender, OpenTK.Input.MouseMoveEventArgs e)
        {
            GameWindow window = sender as GameWindow;
            if (flag)
            {
                //CLEAR THE BUFFER AFTER EACH LINE DRAWN NOT TO HAVE MILLION LINES ON SCREEN.
                GL.Clear(ClearBufferMask.ColorBufferBit);
                float x = (e.X - window.Width / 2f) / (window.Width / 2f);
                float y = -(e.Y - window.Height / 2f) / (window.Height / 2f);
                //THESE FOLLOWING COORDINATES ARE STORED IN AN ARRAY WHEN I CLICK THE MOUSE.
                float x2 = (vectors[vectors.Count - 1].X - window.Width / 2f) / (window.Width / 2f);
                float y2 = -(vectors[vectors.Count - 1].Y - window.Height / 2f) / (window.Height / 2f);
                //DRAWING LINE.
                Draw.DrawLine(new Vector3(x2, y2, 0), new Vector3(x, y, 0f), programDrawing, "aPos");
                window.SwapBuffers();
 
            }
        }

This code works ok except that the line doesn't follow the cursor on time, it always lags behind, I wonder why does this happen and how can I eliminate this problem?!