Is it over the back of your jaw bone where the lump of muscle is? Or further back in the soft area of your neck? If so I would probably say it's a gland or lymp node that is swelling and having a reaction / getting pissed off for some reason. I think I would probably try to stop until you can see a Dr. Or at least snort or something other than IV if you get withdrawals bad.
Do you happen to chew tobacco?
No matter what anyone says it's just a guess and you need to see a Dr. Even if it's scary as hell it's always best to catch things sooner than later.
Does it happen every single time you shoot or have you been pushing long? Some people's bodies just don't like certain things or it might happen when you get a dirty batch or some made a certain way that has traces of something.
How is your health? Do you eat good? Eat plenty of granular salt that contains iodine? Almost sounds like where goiders begin whitch normal happen because of a lack of iodine. If you don't consume much certain drugs cause the body to consume high amounts of nutrients and maybe your body is responding to a severe lack of iodine.
All just wild ass guesses.
See a Dr. And if you can't afford one go to a community not for profit hospital and go to the ER. Takes a long time but they have to treat you and you don't have to pay them back though you will get monthly bills and harass you. But it won't effect your credit ever.
Anyways, see a Dr asap because anything weird, especially from a high dose of drug, may be nothing but better to find out now if it's not
As someone that has gotten terrible medical news many times, it's never worth worrying about something until you get some news worth worrying about. Or else you're just causing undue stress when it probably isn't anything. And if you do get bad news it still isn't worth worrying and you follow the Dr's advice. It's always best to stay positive and if it happens you will know when it's time to worry.
Good luck and do see someone no matter what any of us say.