I tried to compliment her and not sure what went wrong?

I sent my girlfriend a text message letting her know a bit more about how I felt about her and one of the things I said was that I thought she was very sexy. She texted back saying she wans't happy with that comment and I don't know what I've done wrong?! I was only trying to compliment her and not just limit myself to saying what a lovely personality she has and now it seems to have been interpreted strangely. What should I do (if anything)? We're in our mid-twenties and have been dating for two months and recently became an official couple.