What would cause a swollen lymph node by my collar bone 26 yr old female ?

Hard to say.: Any infection, scratch or irritation of the skin, or head and neck can cause the lymph nodes to enlarge. What matters is your other symptoms & how long the lymph node stays enlarged. Consider getting it checked now, so your doctor can check for the cause. When the cause is found and treated the lymph node goes down over the next several weeks. If it stays enlarged, other tests may need to be done