Hey Ambra,
Definitely contact the doctors office. We're not doctors but I'd be surprised if it was sinister based on finishing treatment just last week and your docs doing a thorough exam. It's one of two things in my thinking. It is what you think it is as in a zit/ingrown hair. You were using a lot of creams for the neck burn and quite possibly you have a clogged pore or the like. OR... since you had a neck dissection and removal of lymph nodes (I looked back at some of your OP's), this could be the beginning of lymphedema, or "turkey neck" as we H&N'ers call it. I have a golf ball sized lump under my chin that's sometimes painful and causes pressure. It will eventually ease as my body finds new channels for the lymph fluid to go.
At any rate, try to distract yourself until you can call tomorrow. Also send an email or use mychart.com that JH has. I rarely wait more than 24 hours for responses.
Positive thoughts and prayers
"T"