If your bread becomes very sour and rises badly, then the lactic acid bacteria in your starter and in the bread dough are too strong compared to the yeasts. The lactic acid bacteria mainly produce acid and the yeasts produce the CO2 that loosens the dough. To keep the yeasts in good shape, feed your starter and let it rise. Feeding keeps the yeasts strong and vital. Repeat this 2-3 times in a row, then your sourdough should be fit again. If you use your starter for baking, also make sure that it is in its most active phase: it should have doubled in size and not yet dropped again.
Another reason could be the recipe you used. Recipes that use a large amount of sourdough compared to the amount of flour can quickly become too sour if the starter is not ideally managed.
Last but not least, a significantly too long rising time can also lead to a badly loosened and too sour bread.