How long has she been laying? Have you checked her vent really well? It is pretty common with new layers or ones coming back into lay and should quit for the most part when they have laid for awhile. From what they list as other causes going on with the vent like dirty and irritated/scabbed from droppings or parasites or a prolapse/tear and/or vent picking by other birds, those should be treated and the bird separated until it is healed. http://www.thepoultrysite.com/publications/1/egg-quality-handbook/25/stained-eggs