Pain in back, bone or muscle are one of the side effects that may occur with consumption of Anastrazole. However, these effects are mild and reduces down or are completely gone with the discontinuation of the drug. Also, minor side effects should not bother much to cause discontinuation. Also, pain experienced is not extreme, so it might be some other reason like a reduced bone density, as commonly seen in patients with breast cancer. Go for a bone density test and get treated from an endocrinologist or a PMR physician.