I don't know if it's common to have CD and Lupus together but these diseases are thought to be related to "overactive" immune systems so it's certainly possible that you may have both. My mom has Lupus and I have CD so I'm very familiar with both! Have your doctor do an ANA blood test. It checks for anti-nuclear antibodies which can help with a diagnosis of Lupus. Remicade can also cause Lupus-like symptoms as bektold mentioned.
24 yo- diagnosed with Crohn's in the terminal ileum April 2005
Experienced a year of remission and started another flare December 2006
Remicade (started 04/07), Imuran, Asacol, Bentyl, sublingual B12, multivitamin, probiotics, Fish oil