Hello, You are suffering from a penile shaft lump. On erection you get a large swelling on the left side of the penis shaft. You have not mentioned if you are sexually active or not. Sometimes when your partner has vaginal candidiasis, allergy and inflammation occur after penile exposure to the candida lesions. Get clinically examined by a Urologist as the factor that is causing you symptoms needs to be treated. It is very important to rule out STDs. Swelling also occurs when the lymphatic fluid is trapped in the penis due to a congenital or acquired abnormality of the local drainage system. You have not mentioned anything about getting injured, foreskin and shaft have very sensitive skin which can become swollen, inflamed and painful after an injury. Any tumor in the penile shaft should be ruled out as it may be indicative of penile cancer. It results in forming a lump that can cause pain and discomfort especially during erection. It s rare, but possible. Other medical conditions that can cause such symptoms are Posthitis, Balanoposthitis and Paraphimosis, etc., that may require treatment by surgery. Take care. Hope I have answered your question. Let me know if I can assist you further. Regards, Dr. Nupur K., General & Family Physician