Depends on what you mean by pregnant. Darts and other frogs have external fertilization, so no, the female doesn't get pregnant.
What type of frogs do you have? There are some species with very quiet calls, so is it possible that they are calling but you just haven't heard it?
I hate to suggest this, but it kind of sounds like she is impacted. This is not a good thing. http://www.dendroboard.com/forum/breeding-eggs-tadpoles/63827-impacted-female-heehee.html
English is my second language actually.... sorry about the word choiceYeah frogs don't get "pregnant". Fertilization is external. It sounds like one frog might be sick if it does not eat as much now. How old are they and were they sold to you as a pair or just two unsexed frogs?
Posting a photo of the frog may help us all decide if her size is normal or abnormal, just because you aren't observing the frog eating a large number of flies doesn't mean right away that it isn't eating an appropriate amount, such as when you are not looking or feeding on other food sources in the terrarium.