Just tell her you miss her to. If you don't then she might get mad. girls are moody sometimes. I'm a girl and if I told my boyfriend (if I had one) that I miss him and he didn't say "I miss you to"...
That usually means that she wants to know why u miss her because girls like to hear good things come out of guys which is a good thing. OR it can either be that she doesn't really care if u miss her...