Is it common for a jack russells coat color to change?

by Margo Nova
(Portland, OR)

My puppy, 9 weeks, had a solid white body until 2 weeks ago. Now tiny black spots have developed, she looks like she has freckles on her white body... There were just a few at first, just pepper grains. Now she has about 50, and they are getting more obvious, the size of peas. I thought she was going to be all white!??