I just responded to you and it went into cyberspace....I hate that....
So, if I recall what I wrote...it is my understanding that people with Lupus can develop rectal bleeding due to the muscle tissues being damaged by the antibodies.
If this is the first time you have ever experienced this, than you should contact your doctor who regularly follows you, to report what it happening.
Next time you 'go', take a picture of how much blood and what other slimy things exit. Honestly, the medical staff will not be grossed out and it will facilitate them treating you efficiently.
This site explains the many symptoms, of which you are probably aware.
http://www.clevelandclinicmeded.com/medicalpubs/diseasema...
I'm sorry you are not having a fun weekend :(