User account error

Dear all,
I have problem with domain user.
EX: My account is: tomy
domain name is: gn.san.co; AD server is Windows server 2003
Pc name is: PC1.gn.san.co
when I logon to PC1 with tomy account, normaly my profile is tomy. However someday User profile automatic change to tomy.gn and there is not data here. Old profile still keep all data. Not only this account, but also some other account have same this problem.
What is problem that I have?
Could you help me please?
Thanks!