A CT of the sinuses and an MRI of the brain will do a good job of catching things that involve the sinuses or brain- they may not focus on the other parts of the head and neck (eg skin, throat, thyroid etc), and so potentially could miss some things. It is possible that a lymph node that arises from another source could arise quickly over a matter of weeks, or in the case of possible malignancy, the lymph nose itself can be the primary source, rather than the migration from another source.
That being said, there are other things that this lump could be, so try not to lose sleep about it at this point. Cervical nodes are about an inch behind the bottom part of your ear, rather than immediately behind it.
Your doctor can biopsy this lump if he/she is concerned about it.
Good luck, I hope it all turns out well.
I might also add that I've had my cholesterol test one week ago and it was normal along with all liver functions too.
Just a quick note... my lump was an inflamed lymph node that was next to a nasty bug bite. One week later, the node is normal in size.