I Have Been Taking Linzess For A Couple Years Now.

I have been taking Linzess for a couple years now. Went from taking it everyday to every 2-3 days. (As instructed) because it wasn't working when I took it everyday. Now I take it every couple days. It causes nausea. I just took one and I had a symptom which I never had before. I've never experienced cramping with it but today it was severe cramping and at the end of the bowel movement it was foam-like. What could have caused that??