Margot is correct. Swollen lymph nodes are usually a sign of infection, although they have been known to enlarge (and then go down again) following vaccinations.
Are her ears affected at all? Do her ear flaps have any type of sores on them or is there anything like that anywhere on her face? If the lymph nodes in her neck are the only ones that appear to be involved, it may not be lymph nodes at all, it may be salivery glands that are inflamed, making it APPEAR as though it's her lymph nodes. There is something called juvenile cellulitis, or puppy strangles, that causes an inflammation of the salivary glands and can also cause the sores that I mentioned. She doesn't have to have the sores, though, she can have just the inflamed salivary glands, so that's why you need to get her to the vet to see if her other lymph nodes are involved. Puppy strangles can affect puppies anywhere from 3 weeks of age up to about 4 months, so she is right in the right age group for that.
Ghilly
Are there swollen nodes anywhere else? Check in the armpit area if the front legs and check in the groin area. Any swelling anywhere else? This is usually a sign of infection and I would be concerned seeing this in a young pup. If nothing else, give your Vet a call and see what they recommend.