Have Under Active Bowels, Bloating And Stabbing Pain, Weak And Tired. What Is Wrong?

Hi, I am 30 yr old female, usually healthy. I have always had an underactive bowels. (I have a bowel movement once every 5-6 days). This has never bothered me, I have never had stomach pain or bloating from it, and I eat a regular healthy diet with fibre/liquids. In the last couple of days I have been experiencing pain and bloating in my upper abdomen, lower abdomen ( pelvic area) that radiates in my lower back (dull pain)- similar to labour pains. There is a cramping pain throughout my stomach and then some sporatic stabbing pains in random places of my stomach. The pain is nauseating, but I dont vomit. * I am not pregnant but have had two babies. After about an hour of this pain, I have a bowel movement that is all liquid. I have about 3-5 of them within 30 minutes and its so much liquid that I didnt think I had that much liquid inside me. Its brown, not clear or bloody or dark. Then the pain goes away but I feel extremely weak and tired. Any idea what is causing this?