Has she eaten anything new recently? When I notice soft poo starting, I will feed a half of a plain shredded wheat twice a day. You may also want to pull pellets and feed hay only for the next day or two.
Does she eat Mazuri? This happened to my guy when he was eating Mazuri, I had to do a switch and his poo firmed up really fast. I also fed him half a plain shredded wheat twice a day while his poo was soft as per the advice on this forum.