Can't read cookie after first postback

I have some stange behavior (maybe not, and I'm just stupid). I have a page with a custom control. The custom control has a textbox and a button. The button triggers a Sub that resides in the custom control (I used AddHandler [event], AddressOf [method] in the CreateChildControls method). When clicked, the Sub will create a cookie on the user's computer. The page itself checks for the cookie in the Page_Load event. If the cookie exists, the custom control is set to invisible.
Now, when the button is clicked, the page appears to refresh, and when I check for cookies in the browser, it is there, BUT the custom control is still visible. When I go to another page and then come back, the custom control disappears, as it should, since the cookie now exists. How is this possible? What am I missing here? I did a Response.Write on the page that outputs some text when the cookie exists, but it doesn't output anything after the initial postback. Only after refreshing the pag ...