view the running config of your firewall and look out for rules like this;
ip nat inside source static 172.16.1.1 11.22.33.44
where172.16 or 10.0 or 192.168 can be your private ip range and 11.22.33.44 (respective entry in ur config) is your public IP.
Go through this link.
http://www.cisco.com/en/US/docs/security/asa/asa82/configuration/guide/nat_overview.html