Why have I developed a gummy smile? I have never had a gummy smile before.

Hi Myusername22,
It would be easier to answer you question looking at some photographs.  Having said that, gummy smiles can develop when the gums become inflamed and cover the teeth, making them appear shorter.  Gums could become inflamed as a result of gum disease, medication or other medical issues.  If you have noticed any unusual growths in your mouth or anything is happening in your mouth that you consider abnormal, I would highly recommend that you see a dentist as soon as possible.
Wishing you the best,
Amir H. Motamed, D.D.S.

Okay, this is difficult to answer without a photograph; however, if you now are showing gums but never did before then the most likely reason is related to facial growth. Our facial bones never stop growing and the upper jaw doesn't necessarily grow at the same rate as the lower. It well could be that your upper bone has grown recently. The good news is that as we age our lips grow downward and eventually will compensate for that bone growth.

It is difficult to evaluate your gums and offer recommendations with no photos. I would recommend a consultation with a dentist for a full examination.