I have never had a baby, but my mom was diagnosed with lupus when she was my age and has had FOUR kids! My dad is an ob/gyn, too. What they told me is that women with SLE tend to have very high risk pregnancies. I am incredibly happy for you and I hope your child is healthy and beautiful. What she also told me was that a lot of the weight she put on during pregnancy was edema- water weight- but the lupus made it difficult to shed again. However, she has been working really hard to get her pre-baby body back. Certain foods make her retain water more than others, and certain foods make her feel kind of sick. She's very good at pinpointing what it is that she ate that makes her not feel as good and then she stops eating it for a bit to see if that helps. Usually it does, and some of the weight goes down and she starts to feel better.
I guess what I'm trying to say is, what you eat and how healthy you are and your SLE are all connected. The pain is probably from both lupus and the weight, if it is significant. The foods you consume can really have an affect on your lupus. Keep your journal and make a note of how you're feeling every day and try to make adjustments in your diet based off that. Changing your diet won't cure your lupus, but it will definitely help, and it never hurts to be in good shape!