What Causes Constant Lower Right-sided Back Pain And Falling Haemoglobin Levels?

Hi I currently have continuing lower right side back pain and rapidly falling haemoglobin levels. I have been tested for blood in bowel movements and none found although an internal at my GPs did show blood present. My previous history includes the removal of cystatic tumours on my liver in 2008 ( found to be non- malignant) and removal of polyps from my uterus some years ago. I am 63 years old, fit and normally heathy, practising yoga and swimming regularly. My weight is fine for my height although friends and family say they have noticed weight loss over time. I am female.