Child will not listen

No matter what I do, my 4 yr old will not listen. She has gone from being my sweet little girl to being horrible. She tells me she hates me, doesn't love me, doesn't need me, etc. She will not eat what I make, she will not clean her room. She will not wear what I ask her to wear because it's "too big" or "too small", even though we just bought her new clothes that I know fit her. Right now she threw a fit because her underwear "don't fit". I talked to her and told her she could not leave her room until she put underwear on. She started screaming and crying and ended up falling asleep on her floor, still without underwear. I tried talking to her, time outs, even spanking, which I absolutely hated. We do have a 6 month old, that could be part of why she is like this but she had been doing so good when baby first came. I really don't know what to do. I hate how she is acting. She is rude, mean and lazy. She used to be so nice and polite.