I decided to give my starter, which is maybe only a week and a half old, a go at baking something since it smells good and has a lot of rise. 

I tried this recipe: http://www.wildyeastblog.com/2008/11/05/more-sour-sourdough/ [1] Only 1 loaf, just in case.

The loaf rose good, looked good and smelled good. And at first it tasted good. But after cooling, I noticed that it tasted significantly more bitter. Not sour as a sourdough with rye might be, but just plain bitter. 

Any ideas as to what might cause this? Might my starter just not be mature yet?