Diarrhea What could possibly be wrong? Took Guinea Pig to vet.

I got two new guinea pigs from a local shelter a week ago. Daphne and Delilah. They are both females. They are between 3-4 months old, according to the associate at the shelter. They were together at the shelter, so decided to take them both. I noticed this afternoon that Daphne had diarreah and had a slight amount of puffiness to her face. She was also not very energetic. She was not very energetic last night, but I had finished their new big cage around 11:30 pm, and so thought maybe I woke her up, and she was stressed moving so soon to a new bigger cage. I had to leave for work around 7am this morning and there was no signs of diarreah. She appeared tired, but nothing else looked wrong. I got home around 12:30pm and noticed it. She did eat a bit of orange from me, but I just did not like the looks of her, so I took her to the shelter vet, as per my adoption agreement. The vet has her now, but did not tell me too much. I didn't want to rely on info I found on the web, as far as her health goes. I have not heard back from the vet yet. The vet held her, stroked her, and looked her over quickly while I was there. He said they will look her over futher, and let me know. I am heartbroken over her not being with me. I have fallen in love with her. Has anyone else had this happen with diarrhea and inactivtiy, along with a slight amount of puffiness? I am going to call the vet first thing in the morning to see how she is doing, but am just wondering if anyone might know what is wrong with her, and what the vet will give her? Her mouth and fur is dry, so she was not drooling. She did eat some orange before I took her to the vet.

My other question is: when she does come home, will I be able to put her back in with her Delilah immediately? Delilah seems very energetic, but seems like she misses Daphne alot. She comes when I put my hand in her cage, and is playing and bouncing around, but also seems sad.

Thanks...