Low haemoglobin, muscle weakness, bone tenderness and pain in joints when stressed or folded. What is the cause and what test should I do to analyse?

See a rheumatologist. You have a group of symptoms that may or may not fit together. I would see a rheumatologist. They will listen to all of your symptoms, do an exam, and order a custom set of labs based on your history and exam. Hang in there!