I would probably guess she left and it's not the producers who made her quite. I mean, they could easily write in another pregnancy(they probably wouldn't have to focus too much on it), and most would agree that the show would be worst without Diana, so that's why I think Dana was the one who choose to leave.
This season might have a high chance of being the last season because of this. The show is still entertaining to watch but it's just not as good without Dana/Kimberly, she was a "needed" character IMO.
Although, I guess they could either introduce a new character to replace her or focus more on the kids or something to try to revitalize it.