Ultrasound done. they found a few swollen lymph nodes in my neck. what can cause this?

Usually "reactive": Enlargement from inflammation or infection in the throat or, sometimes, the scalp. These nodes are the ones that normally drain the lymph from these areas.

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.