iphone lock screen display keeps turning on?

I'd start by checking whether there's something that would need it to be awake, like an alarm or an incoming text message. If there's nothing like that, you should head over to your local Apple store and speak with someone there; they'd be happy to take a look at it for you.

Do you have any trouble using it otherwise? The only mechanical cause that comes to mind is a faulty touchscreen; it seems more likely that one of your apps is doing something. Did you install anything new right before you started having trouble?