My cat has large swollen mammary glands, but her belly/sides are still small/skinny like as if she isnt pregnant. why her breasts are so huge?

Hmm… I would take her in if you really think now way is she pregnant

That sometimes happens in queen cats (yes, that is the term) who are not spayed. However, if she's acting like they hurt her, it's time to go to the vet.