What Causes Bone Spurs On The Feet?

I want to know the cause of bone spurs that suddenly appear on my feet, always when I m sleeping. I am 75 years old, not overweight, and have always been super-conscientious about my diet and getting enough exercise, so am puzzled why I have had two bone spurs appear in the past few weeks. They aren t painful, just very unsightly and may cause problems if I wear certain shoes. They were not caused by shoes that don t feel well since I always wear comfortable shoes. I am very active, such as being on my feet long hours but take naps nearly every day.