Inflammed Lymph node. The lymph nodes of the back of the neck drain the lymph fluid from the scalp area. So often when patients complain of this situation we check to make sure they dont have a lesion on the scalp. But otherwise it can be caused by a number of other things. If they dont resolve or get bigger, they should be seen, antibiotics my be necessary.
Lymph nodes. Most likely those are lymph nodes. If you have been "under the weather" recently (sore throat, fever, runny nose, earache, cough), this may be very well explained by your upper respiratory infection. Tooth problems may be another culprit. If none of the above are present, you may seek medical advice as some lymph nodes may be a sign of something serious.
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.