Hello, I'm Dr. Deb and will do my best to help you today.
I'm sorry for this concern for your dog. It sounds as if she may have an enlarged lymph node if this lump you've found is just to the side of her throat and about halfway down from her jaw and neck. A picture may be helpful if you can send one. This LINK
( http://ww2.justanswer.com/how-do-i-send-photo-or-file-expert) walks you through the process.
If this is an enlarged submandibular lymph node and not a fatty mass (aka lipoma which is soft like an egg), then this usually develops when there's something going on in the mouth...usually infection secondary to an issue with the teeth. An infection may not be obvious when you peek in her mouth since they can develop at the tooth root but a dental x-ray could detect such an issue.
I've also seen enlarged submandibular lymph nodes secondary to dental tartar/calculus or periodontal disease which can develop in a dog this age.
If both lymph nodes are enlarged, a problem in the mouth is still the most common cause; however, if others are enlarged, then this may be more serious.
I suppose another possibility might be an abscess or infection since they also can develop suddenly but usually these aren't quite as freely movable.
I wouldn't consider this to be an ER type of situation, especially since she's otherwise acting normally, but I might have it checked out by your vet if she's due for an annual visit soon.
I hope this helps. Deb