Renaming plugin function – still calls old function

Hi

I’ve built a custom plugin. I’m using the following to add a submenu.

function menu_items() {
	add_submenu_page('whatever','Whatever Admin', 'Whatever Admin', 'manage_options', 'whatever', 'whatever_admin');
}

The function whatever_admin used to be called something else. I renamed the function and now when I click the link in admin – it’s still trying to find the old function though I can’t find any reference to it anywhere.

call_user_func_array() expects parameter 1 to be a valid callback, function 'old_function' not found or invalid function name

How is this possible? and how can I resolve it?

Thanks in advance