We discovered the recent stone when we noticed her limping and took her in to have the vet examine for any breaks. The xray was clean for that, but the stone was discovered and flushed under sedation. The vet thought she was moving her back leg oddly too.
We segregated her from her sisters for a week to allow her leg to rest and she was on metacam. I was out of town during this time, but my mom, who was watching her reported her demeanor was good and she was moving around happily in her smaller area. When I got back, she was due to stop the metacam, which I did. I then tried to reintroduce her to her sisters again and there was a little fighting between her and Marmot. They seemed to have worked that out and are all together again, although Marmot does chase her a bit.
Since the reintroduction and the discontinuation of the metacam, Louise isn't moving around a lot. She has a good appetite and wants her foods and hay, but she seems to want to hang out in her hidey in one part of her habitat. She isn't puffed up or seeming pained - she just isn't her usual boisterous self. She is not wanting to be handled much (she was always sort of not keen on being picked up, but she seems a bit more sensitive now). I have noticed her bottom is wet sometimes, but her urine doesn't have an odor, and it isn't consistently wet.
Normally I am off rushing to the vets, but I'm not even sure what to ask about. I am wondering on the healing time of sprains? She isn't limping or moving oddly now, but perhaps the metacam was helping her and there is still some residual pain so she is not wanting to move much? Or perhaps some discomfort related to sludge or the stone removal. She wasn't prescribed any antibiotics following the removal. Her breathing seems good and her eyes, nose are clear.
I took Marmot out of the area, and Louise did seem to explore a little more. So there could be a dynamic issue still going on. But Louise still seemed to want to go back to her hidey even with Marmot away. I could just a sanity check on the situation!
How big is the enclosure they share? Are there private areas? Some people will break up a cage visually by stringing a piece of fringed fleece across it so one guinea pig will not always have eyes on the other.
Are there any piles of sludge? Anything that might account for blockage of urine and her wetness? I think you could be right about lingering pain. Try giving her more metacam for a couple days and see if there is any increase in activity.