If soreness in the neck swellings is not severe, if the swellings are not 'big', if you don't have fever
, you may only observe at the moment. Otherwise, a course of an antibiotic may have to be considered (in consultation with your doctor). Regarding your question 'What could this be', it is difficult to pinpoint it at this juncture (with a short history of only a couple of days and in the absence of other contributory symptoms/signs). The list of possible causes can be a long one. So don't worry at the moment. Keep in touch to inform the development after which a definite diagnosis can be attempted.