What Causes Blood In Stools During Post Sigmoidectomy?

I am 4 weeks post sigmoidectomy with rectopexy. For the past 3 days I have seen dark red/maroonish colored blood in my stool. The toilet water starts turning bright red after a few minutes. I called my colorectal surgeon s office twice today but never got a return call. Tonight I am feeling light-headed and crampy. I m not sure if I should be concerned. I m an LPN so have some basic knowledge, however, not enough to self-diagnose. Thoughts?