Hiding Stressed new guinea pig.

[doragonbat]

Yesterday, I got a new guinea pig at a pet store (here in Portugal it's the only option we have to get a piggie).
My male guinea pig (who was castrated) died recently and I needed a new companion for my female guinea pig. I made sure to get a female aswell. Since she got home she has made soft poop, not diarrhea but not normal poop either. She is very quiet, barely moves but she has drank a bit of water and ate just a tiny bit. I'm leaving her be in her cage, that is covered so she feels as safe as possible because I understand this is stress. But I am a bit worried she might not be okay. Should I take her to the vet today, or should I wait a couple more days so it's not so stressful for her?
I thought maybe introducing her to my piggie already might help but I know it's also stressful to do so and she might be sick aswell, so it's not safe... What should I do?
They also said she has less than a year but her back nails, the pink part, is already very big and under her feet she has them a little dry... I'm not sure if she's that young or not. Help?
Thank you for your help!