It's highly possible she can get pregnant again right now. Keep them separate, consult your vet to have her spayed. You do not want her to get pregnant again so soon after having puppies, as it will truly be detrimental to her health. She has not had nearly enough time to heal and basically de-stress her body.
On a side note, it is odd that she was sold so soon after weaning. Puppies can be weaned at 6 weeks, but they should stay with their mother till at least 10 weeks, especially for small dog breeds.