Where do hard junks come from in my otherwise very soft stool?

Hi! Can anyone tell me what these small (about the size of half of a thumb), very dark and hard junks are in my otherwise very soft (not diarrhea though) stool? At first I thought it's a stone but it turned out to be poop just really super hard and dark in color. Where does this come from? I've had those junks now in 3 days (2-3 per bowel movement) in a row and usually they are accompanied by a string of mucus.
Is it possible to have a partial blockage in the small intestines and you can still poop? And that it would not be detected on a contrast CT scan or during colonoscopy? I've felt kind of sick since beginning of December and had both done 2 weeks ago but both turned out normal. I've had a swollen lower abdomen (from the belly button down) 24/7 since beginning of December and although I used to be a person who would pass gas a lot, that almost stopped and I barely do that anymore. Every time I eat something, my lower abdomen gets even bigger, so during the last week I mainly ate eggs, mashed carrots and very soft fruits. Hard to believe that this kind of food would cause those hard pieces? Or is that possible? Is it possible that I have a partial blockage after all in my small intestines and it started to break lose now?
Or can yeast, bacteria or parasites form these junks?
Help would be very appreciated since I'm getting very depressed due to my situation and discomfort.
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