what happened?

I used my sourdough starter last week the same as I always do,according to the directions that came with the recipe.One cup added to my french sour dough recipe from my Better Homes and Gardens Homemade Bread cook book.Everything looked the same as I placed it in the oven BUT when I cut into it, it was a light golden brown instead of the usual white! I finally figured out it was the mother starter I've been using for over a year now.It hadn't been bubbling like it use to but I never expected that.What happened?Is it still safe to use?