hi there same happened to me if u have limwire on for a while it will take alot of bandwith off you and if you watch movies ect online same will happen i also think if there is any adverts online on a web page or summin it takes it off you duno will the ps3 take it off you depends what you do with it online.
im not with sky but i can buy an extra 4 gig for a fiver if it runs out.sky will probably be the same you should be able to moniter the usage hope tht helps