Lexi may have had a small abscess from the vaccine that drained and is now healing or it may be a location reaction to the vaccine. It is hard to say without examining Lexi first. If your veterinarian has told you it was from the vaccine, then I would follow your veterinarian's instructions for care of the wound.
If it looks red, inflamed, has discharge coming from the area, or changes in any way, it may be infected and will need a reassessment by your veterinarian.
I hope this helps and thank you for using Petco Pet Education Center, formerly Petcoach!