What is her normal diet?
Are there pieces of metal or old farm equipment or old fencing in the pasture?
Any diarrhea or stool down the back legs and tail?
Any trouble breathing, coughing, snotty nose, breathing harder?
Any limping when she does walk?
Is she thin, can you see her ribs?
If she is laying down alot she could have some foot pain, like founder, foot rot, or long hooves or some GI upset. Sometimes over conditioning can lead to founder and some GI issues. It is a good thing there is no metal around, cattle like to ingest that and it can migrate through the stomach. It doesn't sounds like she has pneumonia or diarrhea. Make sure her stool is not black and tarry, that could be a stomach ulcer.
I would probably take her in for an exam. The vet can do a physical exam, take her temp, and listen to gut sounds, heart and lungs, look at mucous membrane color, make sure she isn't dehydrated, and look at her feet. If they can't find anything, she may need bloodwork and a fecal exam (even though no diarrhea).
Does this help?