To give you a brief run down, in Season 4 (https://en.wikipedia.org/wiki/Charmed_(season_4)), Phoebe's relationship to the demon Cole (https://en.wikipedia.org/wiki/Cole_Turner) blossoms to eventual marriage. Her happiness in marriage however, becomes short-lived when Cole becomes possessed by the Source (https://charmed.fandom.com/wiki/The_Source_of_All_Evil) and manipulates her and the sisters to performing a dark wedding. Paige starts to add up to Cole's behavior but Piper and Phoebe are ignorant to it until they become blindsided by Phoebe's pregnancy. Her pregnancy soon gives her demonic powers and there is a belief that the evil of the baby's influence lead her to side with Cole and become his Queen of the Underworld (https://charmed.fandom.com/wiki/Queen_of_the_Underworld).

Ultimately, Phoebe's goodness outweighs the evil as well as her love for her sisters. That love for her sisters is what leads to her eventually vanquish Cole as the source. Throughout the last part of Season 4 (https://charmed.fandom.com/wiki/Season_4), Phoebe mourns over Cole and was still in love with him.

But come Season 5 (https://en.wikipedia.org/wiki/Charmed_(season_5)), Phoebe's feelings for Cole suddenly goes from love to hate.