There is no reported causal association between cancer and fibromyalgia to my knowledge and that I could fing in a literature search. However there is an association between chronic fatigue syndrome and cancer (an association does not establish a causal connection). A recent report analyzed 1361 patient records. While they found no association between fibromyalgia and cancer in patients whose fibromyalgia diagnosis was confirmed, they found that women referred to the hospital for muscle pain and/or tenderness who did not meet the fibromyalgia diagnostic criteria did have an increased overall cancer rate, with an increase specifically in breast, lymphatic and hematological cancers.
Dreyer L, Mellemkjaer L, Kendall S, Jensen B, Danneskiold-Samsøe B, Bliddal H.Increased cancer risk in patients referred to hospital with suspected fibromyalgia.J Rheumatol. 2007 Jan;34(1):201-6.
McBeth J, Symmons DP, Silman AJ, Allison T, Webb R, Brammah T, Macfarlane GJ.
Musculoskeletal pain is associated with a long-term increased risk of cancer and cardiovascular-related mortality. Rheumatology (Oxford). 2009 Jan;48(1):74-7.