Stella's very open with her considering how little time they've known eachother. I would have thought a person with her background would have been more hesitant in oversharing.
she was put in a position where she felt forced to share a secret which hurts her to talk about and sonya responded by saying and doing the perfect thigs for that moment.
and now she's feeling giddy and happy on the basis of that. all of this is probably spilling out of her without her being able to control it. if they had separated after the party and met up again the next day, she likely wouldn't say all that.
but, right now, sonya has the opportunity to really prove herself trustworthy to stella and, if so, she'll have basically forever earned a place in stella's heart and stella will feel more comfortable with her than anyone else.