Looking for advice!!!

We just got our 3 year old an 8 week old dachshund last Sunday. Her first day with us, she was extremely hyper and happy but after a few hours that same day, I noticed blood in her stool. It freaked me out so I contacted the lady I got her from and she told me it couldn’t be worms or anything else because before we picked her up, she had a vet visit and a fecal sample was taken and everything came out ok. The lady told me she was probably just constipated due to stress from being separated from her siblings. I said okay. It still continued so I took her to the vet today and they were just so rude! I gave them paperwork from her last vet visit and the receptionist told me “If everything is ok, why are you here? What do you want” I told her “I literally just told you my dog is pooping blood!” The vet ended up checking her out and said that maybe her food was dyeing her poop red and that it wasn’t blood. Like i said her first day home, she was hyper and happy and always wanted to be around us no matter what..she didn’t eat much to begin with that I noticed but now, she isnt eating really anything at all. She doesn’t care to be around us and she doesn’t have the same energy. Also, I heard it’s normal for puppies to sleep alot but she really has been sleeping so much and on top of not eating well it makes me feel like there is a bigger issue. A lot of people are telling me just to switch her food and wait it out but I’m nervous to do so. Should I take her back to the vet?