I have noticed the same thing too often, even from females who are good friends where there's never been any issue of my intentions or our relationship. I am offended when the female does this because I interpret it as if she's saying or thinking that I'm a creep. I am not looking or glancing at her boobs, and I am not saying anything forward or inappropriate. Most times there's nothing to look at really. I have come to the conclusion that it's not personal and that I must be giving off vibes that I think she is attractive and she is just trying to fend off any such thoughts of mine.