OP: I’m a consultant Radiologist specialising in the Head and Neck at a busy referral centre in the UK so I see a lot of cases like you in my ultrasound lists.
The vast majority of people turn out to have benign reactive lymph nodes – responding to some form of infection (tonsils, ears, flu/upper respiratory, skin (beards/ingrowing hairs) are common culprits). These sort of prominent lymph nodes (medical term is lymphadenopathy) are appropriate bodily responses to infections. Also as another poster said above, by the time they get to have an ultrasound, many have resolved (and I still tell my patients that they are doing the right thing in having me scan them).
A small proportion turn out to have over causes – unusual infections, not lymph nodes but cysts or small fatty growths for example, or cancers. The latter do occur and the nature is entirely variable depending on age, medical, family and social history (for example they were more common in heavy drinkers and/or smokers and are now more associated with some type of human papilloma virus infections). Although a small proportion, these cases want catching earlier rather than later and therefore I strongly urge you to see your GP. This is, statistically, likely to be nothing but everyone has differing risk factors which cannot be assessed over the internet and so please do go see your GP. If you end up seeing one of my colleagues in the UK or even me then (even if the lump has gone by then) you are doing the right thing.
