Most likely just from the teeth being pulled, some is also ingested during the operation. If the poo is just dark and everything, gums, no bloating, etc, then there should be no problems. I would call your vet in the morning, just to check with him. I wouldn't worry, she sounds like she's recovering just fine, just continue to monitor her gums, and incision.
Cheers Quad & Gina
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RIP Megan 3/1998-5/28/2013
