Need advice about my girlfriend?

well i have been dating this girl for about 5-6 months now i am totally in love with her. i'm 21 and she is 17 which is ok for the both of us. she is my the first girl that i have ever had a relationship with. she has been in 4 other relationships but she has never felt this way about anyone. our relationship is going great. we spend almost every day together and we make each other laugh all the time. i like to cook her breakfast and dinner whenever i can. i also like to clean for her too (like her room, dishes or whatever). we like to be intimate whenever we get a chance. but for the past month she has slowly changing. she does not seem as "happy" as she usually is. we dont play around(goof off) as much anymore. and she never wants to be intimate (she doesn't even want me to touch her intimately at all anymore). she tells me she does not know why so im need some advice. do we just talk it out or is this just a phase. thanks for your help