set_flashdata breaks my session

[eluser]Wazzu[/eluser]
Hi, guys, this newbie really needs some help.
I have a simple user login which sets and checks session data for the current user.
Then I have a 'user' controller to edit user's profile.
When I use $this->session->set_flashdata from the controller, my session goes away, so my user needs to login again.
I can't get the reason, it seems that session data gets overwriten when using it.
What is wrong here? How should I do it?

Thanks in advance.