Well, if the turtle had access to gravel in it's previous home, that's where its coming from. RES are known to swallow gravel...different theories have been tossed around as to why they do this, including the possiblility of some eating gravel to aid in digestion and food movement. Even if that's the case lots of people have run into problems with bowel obstructions, so they have taken the gravel out of the tanks. The gravel can stick around in the turtle's digestive system a several months. She's probably just eliminating whatever is left in her. Just make sure that you scoop it out so she can't reingest the gravel. If it persists or the fece starts to look strange, gravel or not, you can snatch up that next round of poo from the turtle and take it over to your vet for a fecal exam. Also, check with the previous home...if she had gravel access, you know where it came from. If she didn't, then you might want to do a fecal just in case.