What happened to Billie Piper's teeth?

Just watched the new episode of Dr Who, Billie has a distinct overbite now, and was speaking much differently than when I last saw her on Dr Who. ( ie. with a lisp) What happened?
Update: To His Nibs: what are you talking about? She's not American, and they don't get the new Dr Who episodes on TV over there so what are you on about??
Update 2: Thanks for all your informative answers, and, sorry I didn't know the new Dr Who was being shown on TV in the States! My best friend lives over there and when the new series came out I mentioned it to her, and she said she hadn't heard of it being over there, but, I haven't mentioned it since, so I... show more Thanks for all your informative answers, and, sorry I didn't know the new Dr Who was being shown on TV in the States! My best friend lives over there and when the new series came out I mentioned it to her, and she said she hadn't heard of it being over there, but, I haven't mentioned it since, so I guess unbeknownst to me it made it over there in the meantime!
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