Potty training advice/ attitude

So I have 3, and 4 year old daughters. ( Ten months apart) and currently pregnant with number 3. They are very different personality wise. My 4 year old was potty trained by 2, listens to me very well, has occasional fits when she doesn't get her way( normal for her age) and likes to occupy herself. my 3 year old is going to be 4 next month and she will only poop on the potty. I don't understand why but she will not pee on it(she used to). Her attitude lately has gotten much worse, not just with potty training. she refuses to wear underwear. I put undies on her and she looked right at me, peed on the floor and said "I said I wanted a diaper." My jaw dropped. I've tried rewarding her, being super excited and happy when she goes on the potty, sticker charts, letting her have privacy, telling her about going to preschool and how after she learns she'll be going and making new friends, just straight up having her wear underwear hoping she doesn't like the feeling of peeing but she just doesn't care. I don't understand why she will poop but not pee. I don't know what to do, I obviously don't want to yell at her but at this point I don't think it's accidental, like I said she will look at me, and then do it right in front me. She was peeing on it but just stopped and now refuses. She also has been having the worst attitude about literally everything, whining all day, she needs my constant attention I can't even go to the bathroom, recently she started yelling at me at the top of her lungs, I've tried time out, changing the subject, just talking to her. And we were in the store recently, I let the girls walk next to me but my youngest kept trying to run away, I told her if she wasn't going to listen she would have to sit in the cart, she threw herself on the ground and started screaming, I told her if she didn't stop we would have to leave. Well she didn't stop, so I went to go pick her up, I didn't even touch her yet, and she's yelling I'm hurting her, she was screaming it all the way out the store. I was so embarrassed! But we left. I have no idea where this is coming from. I don't know where shes learning this as I'm a SAHM, I don't know what to do at this point, but she's changing and doing it very fast and I have never had these issues with my first. Am I doing something wrong? Is this something to talk to her doctor about? What else can I do that I am not? Thank you for reading all of this, I really appreciate any and all advice.