Thank you for your question and for visiting the Family Practice Forum.
I can tell you that if there is any inflammation or infection of the skin, nasal passages, throat or ears (or essentially anywhere in the "head and neck" region), you may get the enlargement of one or more lymph nodes. Without a doubt, the presence of acne quite commonly is associated with lymph node enlargement in this area.
There are certain characteristics which make the presence of a lymph node LESS worrysome. [smooth, soft, and freely moveablve (when you feel the node beneath the skin) is less worrysome VS. fixed, rough and hard).
The size in and of itself makes me worry less, as does the fact that the ultrasound did not cause reason for concern.
To make a DEFINITIVE diagnosis, you need a biopsy. This may be the best plan of action to take if you continue to worry about this despite your doctor's reassurance.
I can tell you that my gut feeling here is that this is nothing more than a reactive lymph node which is present due to the skin inflammation commonly seen with acne. However, most doctors would agree that despite the physical characteristics of this lymph node and the reassuring comments you received following the ultrasound, that if the lymph node persists, OR if it enlarges it should be biopsied.
I wish you well!
Dean M. Tomasello, M.D.