There are 3 swollen lymph nodes on the left side of my neck and the doctor can't find a cause. what could it be?

May need a biopsy: Swollen lymph nodes without a cause need to be investigated. There are many reasons lymph nodes can enlarge but it will likely require a biopsy to figure this out.

Pursue it: Inflammatory response is a common response of ln with enlargement. A recent viral infection, skin eruption of head and neck area may be the cause. There are malignancies that may present with enlarged ln. Treatment may vary from watchful waiting, antibiotics, to needle biopsy of ln. Point is to have it followed by your doctor until it is resolved.

Lymph nodes: Inflammatory, infection, malignancy. Do you have a young cat at home? Also, you should be seen by an oral surgeon to fully evaluate your mouth.

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.