As others have said, this is all about body language. Unless she's stopping abruptly, or making the circle small on her own or trotting AT you, she has likely asked you several times if she could stop, and you said yes! She has figured out that when she's in the middle with you, there's less work. So she slows down and angles towards you (swings her hind end away) asking if she can come in, you don't push her out back on the circle and forward, so she comes in. No wonder she seems confused when you try to send her back out when you've already told her its fine to come in. Either that or she doesn't like you on that particular side, and so she's moving it away from you.
For now you should always carry a whip with you. Not to smack her, but as an extension of your arm. It's all about timing, and the trick is to recognize when she's starting to turn in, and catch it before she's facing you. But this will be hard to recognize at first until she actually turns towards you. Before you start lunging, do some ground work. Get her moving away from you (backwards) when you walk towards her chest/shoulder. Get her so that she'll move away with you just walking towards her with purpose/while clicking (or whatever your "move it" cue is). When you're lunging her and she turns into you, push her back with your body language to stop her from coming in more. Push towards the inside shoulder to encourage her to move the shoulder away. At the same time swing the lunge whip at her barrel on her inside side. This combination will tell her to move her shoulder away from you, but to go forward, so she will move forward on the circle instead of moving forward towards you. She might just keep turning to face you if you're not firm enough. Don't get stuck trying to walk around her to get to her side and she keeps following you. Be very assertive about pushing her back, and make contact with the whip.