I suffered from IBS-C for over 5 years years. I was fit, exercised daily, and ate healthy.
It became so bad, I had only about 10 foods I could tolerate - if is ate anything else, my digestive system would shut down for a week as if I had surgery. It was miserable and I lost too much weight.
I don’t know what happened but after a minor bout of Covid and week of ivermectin in 2022, I recovered. I can eat almost anything now.
I think there is a lot they don’t know about IBS.
Less stress and less greasy food helped me a lot.
Sounds like you had it bad. So maybe the ivermectin helped?
… maybe was parasites??