Can a piercing cause inflamed lymph nodes behind the ear?

Piercing: Yes, if there was a mild infection. Just the trauma itself shouldn't give you too much of an issue. If the earlobe is getting more painful, see your doctor.

Monitoring Centers: Lymphatic system absorbs/carries things too big to go into the arteries/veins like bacteria eaten by white cells and digested food. Lymph nodes "taste" the lymph for anything bad like bacteria/viruses/cancers, etc. When it detects a problem, it alerts the immune system and your body reacts. They enlarge in response to the inflammation they create when they react. They shrink when all is well.