Could be a thyroglossul cyst, a cystic hygroma, a lymph node that is blocked, it could be a normal sebacceous cyst. Or a cancer of some sort. Who knows! As the above poster says, you need to go to the doctor to ask the question. We are not medically trained and even if we were, without seeing you in person, it would be unlikely to know.
Go to the doctor.
Hello,
we can't recommend or diagnose on a patient to patient forum. However, from what you describe a trip back to the doctor is now urgent.
Let us know how you go.