Server :: Internal DNS Setup That Serves LAN?

I want to have an internal DNS that would serves our LAN and I also had external DNS server (located on our ISP). I have configured my internal DNS .. by adding new A record named test.mydomain.com (for example) and I knew that test.mydomain.com also exists as A record on external DNS server. When I ping test.mydomain.com .. it still return from 202.x.x.x (it should be return from 172.16.250.1? It seems like my internal DNS config below not configured properly..

// named.conf
// Provided by Red Hat bind package to configure the ISC BIND named(8) DNS
// server as a caching only nameserver (as a localhost DNS resolver only).
// See /usr/share/doc/bind*/sample/ for example named configuration files.

options {
listen-on port 53 { 172.16.250.2; };
listen-on-v6 port 53 { ::1; };
directory "/var/named";
dump-file "/var/named/data/cache_dump.db";
statistics-file "/var/named/data/named_stats.txt";
memstatistics-file "/var/named/data/named_mem_stats.txt"; .....