From what I have read about desmoid tumors - they occur in children and pregnant women. I never knew they were linked to lupus.
I still think it warrants further investigation - either way.
An ultrsound or xray is warranted. Most tumors are biopsied to ensure that they are benign.
I'd check back in with your doc and let him know that it is still bothering you to see if he will pursue further testing. If not, maybe time for a second opinion.
Wishing you the best!
Post back and let us know what happens.