How to turn a friends with benefits into a serious thing?

I had sex with a girl after 15 minutes I met her, then I saw her again a few times, and the same thing happened! now she is barely texting me and asks me almost no questions to get to know me, We have the plan to go on a little trip soon me and her but I feel like things are not going to work out, she is very nice replying to me when she does but I feel like is me the one putting the efforts, she is a very good girl and maybe she is ashamed of what she did, I don't know but I like her!

How to turn a friends with benefits into a serious thing?
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