Welcome to the gastroenterology community! I don't believe this is a side effect of amitiza. I would recommend seeing a doctor about this.
My daughter has been on Amitiza 8 mg but today it was increased to 24 mg because of chronic constipation. She is only 9 years old. I had to give her an enema and it didn't work. I can't figure out what keeps binding her up.
As your question is a little bit different that sskakj's question, I would recommend posting your question as it's own post.