Hi Nikkie is dead right because for you to go to the trouble to post on here you must be worried.There is no magical cure you need to see a professional as soon as you can you could never forgive yourself if something happened to Um him/her.?
Good IuckI hope he/she is ok.
Where is the bump?Did you take him /her to the vet? Is he/she a big or little dog?How hard was the impact? You did not give any info all we know is your dog got hit by a car,you should have taken him to vet being hit by a car is horrible your dog needs a full vet examination something bad can be going on internally the dog need to see a vet.