The good news is 1. your dog is young, and, 2. only one lymph node is enlarged. This does not totally eliminate the possibility that it is cancer but it would be farther down on the differential diagnostic list than if your dog was 12 years old, or, if all lymph nodes were enlarged. Lymph nodes can also be enlarged by infectious, inflammatory, or even traumatic causes.
To best relieve your worrying a fine needle aspirate (needle biopsy) could be performed immediately on the enlarged lymph node. The needle biopsy is an excellent diagnostic tool. This procedure does not require anesthesia, can be performed in an office visit setting, and is fairly inexpensive. It is performed by inserting a small needle into the swollen lymph node and extracting some cells. The cells are placed on glass microscope slides and are viewed under a microscope by a pathologist, or other diagnostician. Good cellular samples usually yield excellent results. Occasionally a sample is not diagnostic and the aspirate must be repeated, or a surgical biopsy must be performed. The results of the aspirate test usually take less than one week.
Please also request that the vet examine your pet's teeth.