Can stress cause fibromyalgia during pregnancy?

Yes. Unfortunately, the truthful answer to the question is yet, but in many cases, we will see many women with fibromyalgia and certain other auto-immune disorders actually improve during their pregnancies. For unexplained reasons, joint pain, muscle pain, fatigue, and other symptoms can improve during pregnancy. However, there is a risk for a relapse in the post-partum time period.
Not stress. Stress won't cause fibro but it will agra ate it but there is an association with pregnancy likely due to musculoskeletal and hormonal changes. I have seen many cases after prolonged labor as well.