Hello Pamela, I'm Dr. Deb.
I recently came online and see that your question about Tom hasn't been answered. I'm so sorry that you've had to wait for a response,but if you still need assistance, I'd like to help if I can.
I'm assuming when you said he was sick this morning, it means that he was vomiting. Since it doesn't take much to trigger a cat to vomit, I'm not certain if this symptom is related to his enlarged lymph nodes or not.
If he seems to be active and is interested in eating, then I might not be too concerned about one episode of vomiting unless it continues.
You could give him over the counter Pepcid AC (Famotidine) although it's sometimes difficult to find in the UK. The dose would be 1/4th of a 10 mg tablet. Another option would be Omeprazole at a dose of 1/2 of a 10 mg tablet once a day.
I'd also withhold food for 8-12 hours and then offer a bland diet of boiled chicken or human baby food (but avoid those with onions or garlic in them).
As to his enlarged lymph nodes, if only the sub-mandibular lymph nodes (the ones in the neck) are enlarged, then I usually suspect that there's something in the mouth/head area which is stimulating a response....chronic stomatitis, gingivitis, periodontal disease, infection, for example. Sometimes it's obvious as to the trigger but sometimes cats need to be sedated for a more thorough exam.
Other possible explanations for enlarged lymph nodes are as follows:
1. Bartonella (LINK) which is a bacterial infection responsive to several different antibiotics such as Doxycycline or azithromycin. This disease is often contracted through fleas.
2. Retro viruses such as Feline AIDS or Feline Leukemia. Even though the tests may be negative for Leukemia, this organism can hide in the bone marrow, making it difficult to diagnose at times.
3. Lymphoma which is as you know, unfortunately, cancer. This isn't always the easiest diagnosis to make, believe it or not, especially if only a needle aspirate was done and the cells evaluated. I should point out that aspirates are not always diagnostic, unfortunately. It's often better to submit a biopsy or even better, an entire lymph node for evaluation if there's any uncertainly about the results.
4. Idiopathic which means that we can't identify an underlying cause. The lymph nodes can take weeks to months to return to normal.
5 Mycobacterium (a form or bacteria) or other unusual organisms but these are going to be difficult to diagnose without special staining of tissues (requiring more than just an aspirate).
While I can understand your concern for Tom, his enlarged nodes don't necessarily have to be anything serious or life threatening.
As to his overgrooming and licking himself, I'm having a difficult time relating this behavior to submandibular lymph node enlargement unless he has had a flea problem. Then #1 above may explain both symptoms.
There are other reasons why a cat would be itchy ranging from seasonal allergies to pollens, grasses, trees, etc or allergies to food (usually food they've been eating for a while) ....but in most cases, if the lymph nodes are going to enlarge because of inflammation, I wouldn't expect only the ones in the neck to be affected.
I hope this helps and gives you other options to discuss with your vet; again, my apologies for the delayed reply. Deb