My homemade sourdough bread tastes bitter after letting it stay for one day

I do sourdough bread at home, and through the process, I've tasted lots of not-yet-baked breads. Also, tasted someone else's sourdough bread, which was quite good.

Now, I'm in the stage where I can properly prepare the dough, knead it, and baked it enough so that at the end, I get a good baked bread with a lovely crust, see my latest bread in here, and its inside

However, after cutting the bread, and waiting it for a day or so, the inner side of it tastes like as if it unbaked; it has a bitter taste like half baked dough. What is the reason for that ? After cutting it, I wrap a towel around it so that is doesn't lose it moisture, which in general does not help much for some reason.

I'm using mostly - around %90 - strong white flour, and corn flour with whole wheat sourdough starter.