You mistook a minor difference in lymph nodes (possibly due to allergies per the doctor) to be cancer but the cat scan proved your neck is normal. It doesn't appear that you accepted this, and are still convinced your prior ideas must be correct since you have listed them all.
Now you think they are swelling again and without seeing your doctor, you again fear undiagnosed cancer. I suggest you see doc again as he said to do if it swells again, however his previous diagnosis is allergy so even if they really are swelled then likely that is the cause now.
There are 5,000 disease conditions that a human can have, so their isn't enough time in the day to be doing body checks for everything or much of anything for that matter. I never check my lymph nodes so if this turns out to be another false alarm I would suggest you find a way to stop worrying about a cancer that has not been diagnosed. If you can't stop worrying about cancer but there is nothing wrong this time when you see doc, a therapist might be useful.
Anxiety about getting cancer is fairly common. Health anxiety causes people to look at everything on their body or feel every twinge within and think something serious is happening because of it. In actuality, it is all likely normal. You've been checked by a doctor. They found nothing. You can have a good thorough physical to be on the safe side mentioning your concerns. If at that time, the doctor does not suspect any abnormalities, you know that it is time to then begin discussing your anxiety. As in anxiety would then be your issue to deal with like you would the symptom your bump you are worried about. good luck