I had and an still having almost identical situation. Had diarrhea with COVID and then began having constipation and passing nothing but pebbles. I began having pain under left rib cage and on occasion lower left abdomen. I've had CT scan of abdomen and they ruled out any physiological problems and just told me I was severely constipated. "Take fiber and drink more water". Despite going to the bathroom almost everyday, I'm still passing pebbles and still experiencing pain under left rib cage.