It's quite difficult for Sage to express her feelings. She, like MС, is new to love and relationships, despite the fact that she has been dating Chad for a long time. In my opinion, she is subconsciously afraid of falling in love and being abandoned, so her relationship with MС is quite cautious until the end of episode 8.
She is actually very similar to Quinn, only she does not have such a strong tsun-tsun side as Quinn. Quinn has a complicated background, she just simply do not trusts people. Sage is friendly to everyone, but it is difficult for her to move from the "friends" stage to the "relationships" stage, because this is completely new and unusual for her and she is afraid.
It's not so obvious in episode 9, but at the end of episode 8, when she writes MС, you can see how nervous she is about the fact that MС is her boyfriend.
If she is rejected, on the contrary, she sighs with relief, because she probably started falling in love with him and she was embarrassed. Sage, after breaking up with Chad, is afraid of being rejected again and this is a very delicate moment. She really has a rather gloomy background, based on the fact that she goes on a thin line and is ready to break at any moment.