Welcome to the wonderful world of guppies.  

When you are mixing the males and females, a good rule of thumb is to have at least 2 females to every male. This will reduce the stress that is brough upon her from the constant chasing from the male. By adding another female you will then be able to remove your other female from the breeding net.  Putting her in this early will only stress her out.  

I'm guessing that she is at the early stage of her pregnancy.  I only say that because 95% of the time you purchase females from your LFS, they are pretty much already pregnant.  As she gets further along,her belly will get bigger, and her gravid spot will get darker.