My Lips Go Higher Exposing The Gums, Why Is This?

Hi,up to the age of 34 my top lip sat perfectly on the top of my upper teeth when I smiled, not exposing the gums. After 34 ( I am currently 36) the top lip now goes up around three quarters of a centimetre on each side, exposing the gums. It is only the part of the lip which sits directly above the two front teeth that seems to have remained in the same position. What can cause this. When my lip are closed it doesn't seem like I've had thinning of the top lip.

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