Greetings, and thanks for the question.
As long as your pup is doing fine otherwise, I wouldn't consider this an emergency situation. The fluid lump that you have noticed most most likely a buildup of serum from inflammation due to the trauma of getting hit by the car. Another possibility would be an abscess, which is a pocket of pus due to bacteria getting under the skin.
Although not an emergency, I would schedule an appointment with your Vet first thing in the morning to have the lump checked out, avoiding any discomfort to your pup or enlargement of the lump.
Please let me know if you have any further questions.
All the best!