So sorry to hear your puppy isn't himself and that he is anemic. Anemia is definitely different than the condition of Autoimmune Hemolytic Anemia (which is IMHA). Anemia is the broad category of having low red blood cell counts and in young puppies the most common consideration is parasites, especially hook worms. Fleas and other ecto or skin parasites may also be a cause. It is possible that she came to you with worms or even was anemic from early on if the breeder was battling fleas. IMHA is testable with an autoagglutination test, and is almost always in mature dogs. IMHA has variable success rates, although I find I have had many many more successes than the internet would lead you to believe. Admittedly, some cases are hard and a few even require heroic efforts such as chemotherapy drugs and even spleen removal surgically.
Puppies are growing so rapidly that it is harder to discern their normal, and if the anemia was just slightly, it may be wise to simply recheck in a week or two to ensure the anemia isn't worsening. In the mean time, ensuring that broad spectrum deworming medicines at your veterinarian's discretion are being given, even if the fecal evaluations are normal. Larvae parasites will not lay eggs and thus will not be seen on fecal flotations, but can still be voracious blood suckers and cause pathology. Occasionally we'll see blood or mucous in the stools, but not always.
It is good that we're happy and content, and if we're eating and playing, I would think your vet would want to check or recheck a fecal, and probably deworm regardless (the fecal evaluations are not 100% but if you do see worms, it will set your heart at ease and allow for a focused and persistent attack). If the anemia is profound or worsening over time, then checking for immune mediated causes would make sense, but again, it is not my experience that this is common in the young animal.
We don't see the nutritional causes of anemia in dogs that we do in people as long as we are feeding a quality pet food. We also aren't going to suspect cancer in such a young animal, so parasites are your most likely cause, whether current or historical.
Please let me know if there is any more information that I can provide to help you.