Lab in heat

Question
My 13 month old Red lab started in heat three weeks ago, and I noticed that a week ago her nipples were more prominent.  To my knowledge, she has not been around any other dogs, and have not caught any dogs in my neighborhood hanging around the fence.  Is this a sign that she may be pregnant, or just a symptom of being in heat?  

Answer
are you planning to breed her at 2 years old?  

If you are not planning to breed you increase her chance of pyometra (infected uterus) and mammary cancer by 10 fold.

what your dog could be experiencing is false pregnancy.

a typical pregnancy cycle for a dog is 58-63 day.