It might be that you have just over fermented the culture, or that you have some form of a lactic acid producing bacteria present that is giving you the very sour/acid taste where as a different lactic acid producing bacteria strain might give a totally different flavor profile. This is one of the things that can happen with these natural ferments, it is also what makes working with them so much fun and so rewarding at the same time. If it isn't over fermented, and if you are not using too much, you might need to start over again and hope for better luck the next time. One other thing, are you sure you used the correct amount of inoculant when starting your culture? If you don't use enough to inoculate with, you may not get enough of the desired bacteria and yeasts to propigate the microflora in the culture, and hence you run the risk of ending up with something totally different.
Tom Lehmann/The Dough Doctor