Hi there and thank you for your question. I am a licensed veterinarian and would be happy to assist you. I am concerned about the weight loss despite eating well. The weight loss combined with the distended and firm abdomen make me concerned about a few possibilities:
1) She could have a cancerous mass in her abdomen, and with cancer they can lose weight despite eating well.
2) She could have heart disease, which can cause the abdomen to fill with fluid and then the abdomen can appear large and firm. Heart disease can cause weight loss despite a good appetite.
3) She could have heartworm disease if she has not been on heartworm preventative. This can cause fluid to build up in the abdomen and can cause weight loss despite a good appetite.
I doubt that this has anything to do with being hit by a car, but it is hard to say until some diagnostic testing has been performed. I recommend starting with a physical exam to listen for a heart murmur, and then potentially abdominal x-rays and see what that shows and then go from there. Hope this helps... best of luck.