Lymphatic pain in the armpit may be caused by the following reasons: First, it may be caused by soft tissue infection around the armpit or the upper limb, which spreads to the lymph nodes through the lymphatic vessels, resulting in painful inflammation; second, it may be caused by malignant tumors in the chest or breast, and the tumor cells spread through the lymphatic vessels, resulting in swelling of the lymph nodes in the armpit. In this case, you can generally go to the hospital for ultrasound examination of lymph nodes, which can basically distinguish the pain. If malignant tumor is highly suspected, local lymph node biopsy can be done to clarify the diagnosis.