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Painless swelling at the elbow is generally due to sterile (non infective) fluid collection in the bursa of olecrenon process on ulna bone of the hand. This process is present at the elbow and a bursa or sheath covers it. When sterile fluid collects it is painless bursitis. This bursitis can become painful if there is infection.
Generally there is a history of elbow injury of some sort in the past, but it is not a must.
Treatment is by aspiration of fluid and corticosteroid injection.
Please consult an orthopedic specialist as a confirmed diagnosis cannot be given on net.
Also since you have multiple joint pains consult an orthopedic specialist or a rheumatologist to rule out fibromyalgia, hypothyroidism, low Vitamin D levels, Low Vit B12 levels, anemia, Hasimotos thyroiditis, gout (can sometimes cause multiple joint pain), rheumatoid arthritis, osteoarthritis, SLE, electrolyte disturbance particularly those involving calcium and magnesium, parathyroid gland problems, chondrocalcinosis and chronic fatigue syndrome. Menopause or menopause like condition caused by PCOD is often associated with Vit D and calcium disturbances and hormonal disturbances causing multiple joint pain. Please talk to your doctor regarding these and get yourself examined accordingly. Take care!