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It is perfectly normal for a dog to bleed after whelping. Bleeding can last up to 5 weeks though often it stops earlier. It starts out bright red and gradually fades to a brown, light tan or pink until it stops completely. If your dog was eating and drinking and caring for the pup, most likely she was done delivering. If she was not eating and drinking, etc, then she may have a puppy stuck. If there is any possibility that there are still puppies waiting to be born, then she needs to be seen by your vet. The average size litter for a pit bull is 4 to 10 so a single pup litter would not be normal.
If your dog had one pup and then more than an hour or so went by before having another or she gushed blood or fluid and no pup or afterbirth was seen, then that might indicate a stuck pup. This would be considered a difficult labor. You can read about these below.
I would plan on having her seen this morning at your vet to be on the safe side given her breed and a single pup. Here is a site on whelping and will be helpful as it goes over raising pups as well.
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