Well, she might. There can also be other reasons why. Go check with your vet. It might be more serious than you thought. This happened to my friend's dog once.

 I sounds like she is expecting. Congratulations!

 Well I can' t really say because I have a dog who nipples were as you described and she was never pregnant. At least not until seven years later when she had her first batch. Who knows maybe its normal. But if she was in heat and got lose there is a very good possibility that she is pregnant.  Good Luck.