My first thought would be coccidiosis - causes liquid manure. It's a parasite they pick up from ingesting manure from an infected animal. Can be from eating off the ground which has or had manure on it, or licking other cows with manure on them or whatever. Around bale rings can be nasty.
Anyway, before you run the heifer to the vet, try to collect some of the runny manure in a jar and take that in. They can look under the microscope to see the parasite. You may also want to isolate her as she could be spreading it around for others to be infected. Might consider preventive treatment for others - pour Corid in water.
At this stage, too late to treat the coccidiosis in this heifer, though may need to treat any complications. Hopefully she's still eating and drinking. After the infection she'll be immune. Other cows may already be immune if they've had some degree of infection, or maybe they weren't exposed to as much as she was - different eating habits?
Younger animals more likely to be infected due to lack of immunity.
All assumptions though, until you get a diagnosis.