To me 10 days after mating is a bit early to notice any signs of pregnancy. With my girls it is usually about 3 to 4 weeks before you can notice any signs, I have always found the best indication to be a clear/opaque discharge at around 5 weeks but before then it can be quite difficult. With regards to you feeding her differently now I again usually wait until they are 5 weeks pregnant to change to a puppy food as they dont need anything extra before this time.