My 6-week-old kitten is lethargic and sleeps all day with bloody poop?

Hi and thanks for using Petco Pet Education Center, formerly Petcoach! Izzy is very cute. It's quite likely that the blood you are seeing is from intestinal parasites. Puppies and kittens should be dewormed starting when they are 2 weeks old. The deworming should be done every 2-3 weeks for at least 3-4 times to clear out as many parasites as possible. Some of the intestinal worms are blood suckers so they suck blood from the lining of the intestinal tract. This can lead to anemia or low red blood cell numbers, which can cause low energy, lethargy, sleeping more. Other reasons for blood in the stool could be intestinal infection, dietary change or dietary problem, foreign body ingestion, toxins, etc.
You should take Izzy to your veterinarian so they can assess her, advise some fecal tests, other tests if needed, and treat her accordingly. They can also advise you on her preventive health care so she gets a good start out in life. Good luck with Izzy.