Your mother is right.

Fish oil can thin the blood a bit, but most of the time this doesn't matter. It does matter for people in the hospital who get treatment to prevent blood clots (such as heparin injections). Fish oil may increase their risk of bleeding.

Taking high doses of omega-3 fatty acids (a key ingredient of fish oil), can cause nosebleeds, blood in the urine and even stroke due to bleeding into the brain.

Besides people who take blood thinners, others at risk of bleeding while taking fish oil include those who have bleeding problems (such as a blood disease or a bleeding ulcer) or people who are having surgery.

The overall risk of using fish oil is probably low. But doctors are concerned about the potential interactions of over-the-counter products like this with other products and with prescription drugs. This is especially true for people who take many medications every day.

The benefit of fish oil for joints seems to be modest at best. By the way, the potential benefit of fish oil for joints is probably not as a joint lubrication. Fish oil has a mild anti-inflammatory effect. By reducing inflammation, fish oil may be helpful for the pain and stiffness of arthritis.