Well, she is really too young to be pregnant. I know she can become pregnant but so can a 13 year old girl but it still hurts their body.
She might not be thin at all, she could be in the healthy weight range but you're just used to seeing overweight dogs.
if she is pregnant you need to get it terminated, it will shorten her life by about 5 years if she has a litter that young.
you should be able to feel her ribs easily through a thin layer of fat, you shouldn't be able to fell her pelvis (two lumps on either side of the tail). it all depends on the breed.