The former X Factor star said that she thinks the age gap between the pair, who have been rumoured to be an item for several months, is too extreme.
Ferguson, 25, previously dated Styles's One Direction bandmate Zayn Malik, but the couple split after four months together.
"I think there's something not right about Caroline and Harry's relationship," Ferguson told Fabulous magazine.
"For me and Zayn it was different, because he was 18 so at least he could drive over to pick me up, we could go for a drink in a bar together and he was old enough to make his own decisions. But Harry is too young for that and there is so much more of an age gap between them."
Reflecting on her own break-up, she added: "One day I just woke up and thought, 'This isn't working for me'. But when I meet a man I want to feel like he is with me 100 percent. There shouldn't be any messing around... I'm not saying Zayn cheated."
Ferguson recently announced that her next single will be 'Too Good To Lose', taken from her debut album Heaven.