Where does all this poop come from?

I had a severe abdominal pain the night before yesterday to the extent that I was screaming. I took an antispasmodic and then a painkiller injection. Things calmed down. However, it recurred yesterday with severe tenesmus. I called my GI and told him I'm not constipated but this is something weird. He told me it is because of inflammation of the rectum, and ordered me to take an antispasmodic injection and Pentasa enema as it would ease the inflammation. The enema relieved things but after 2 hours I had explosive , v painful watery D. This got repeated after 3 hrs and I woke up from sleep rushing to the toilet with this again v painful watery D and now again after other 5 hours and there is also that lasts for more than an hour after every bm. I have proctitis and take usually the Pentasa enema but it had never resulted in D. I had 2 bms the day before yesterday, yes they are just 2 and not 7 or more as I'm used to everyday. I also remember that I vomited my lunch. So,where does this quantity (sorry for the graphic). I'm just wondering.
Thank you.