Why is he suddenly acting so cold towards me?

There is a guy a met last month.We were flirting with each other very often that things went quiet fast.Every night he will text me and lovey dovey conversation.He is younger than me 2 years.He says he doesn't mind..all of a sudden 3 days ago he sudden act cold towards me.No more darling etc..no more flirting..speak to me so formal..-.-..no more kisses etc..y?what the hell did I did now?