What could cause only the lymph nodes on the left side of my neck to be swollen (2cm) and have a tender feeling especially to touch, that an ultrasound wont show?

Lymph nodes: It sounds as if there is some source of inflammation on that side. Either dental, tonsillar or other oral structures. Lymph nodes fight diseases for a living and they get large and tender while doing so. This should resolve when the source of the inflammation recedes or is treated.

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.

Enlarged neck node: Although infection in sinus and throat can lead to enlarged neck nodes, biopsied will be needed if it persists to rule out lymphoma and other head and neck malignancy.