Take what I'm about to say with a grain of salt, coming from a bonafide misanthrope.
This girl doesn't need a reason to shit talk. People shit talk for all kinds of reasons. They shit talk because deep down, they might feel insecure about their relationship with you or they're bitter because you drifted apart. They shit talk because something about you might threaten them deeply and they don't have the balls or the maturity to confront that part of themselves. They shit talk because maybe you stepped on a proverbial toe (although in this case, it sounds impossible) and they don't have the same aforementioned balls or maturity to talk to you about it to sort things out (and quite frankly, most people don't). They will shit talk a person because everyone around them is shit talking the same person and they want to feel included. The permutations for shit talking reasons might be infinite and there is absolutely no way to control this - this is the bad news for many of us who are sensitive to interpersonal rejection, so I totally sympathize with your feeling. I have been mercilessly bullied by coworkers for absolutely no reason at all and sometimes they did it only feet away from me, within earshot - and fully knowing it was within earshot. Some of these people lived rent-free in my mind for years.
You have two options - 1) you can talk to her and try to mend whatever rift that seems to have been forming between you, or 2), you can say "fuck her" and move on. I'm sure others here will chime in on the potential backstabbery that might happen if you do try to befriend her and I have to say based on your story, she's not winning any points in the "sterling character" department so far. So obviously you should be careful if you opt for the first, or at least find a way to detach emotionally if things go south.
If you want to move on, you may need to ask yourself why you want to earn approval from someone who doesn't like you? Will approval from this one random girl from your past truly impact your life if obtained or not obtained? Yes, it is human nature for us to want to be accepted, liked, included, but it should not interfere (further) with your self-image or self-esteem. And you have to do your best not to allow some shit talker - whoever they are - take that away from you because you deserve better.
Take it from someone who wasted a lot of time feeling badly about being rejected by other people. While it's good to reflect the best parts of ourselves outwardly onto our relationships with others, we still only have a finite amount of control in the matter of how others perceive us. If someone talks shit, remind yourself that you are worthwhile, have value, and that there are others in your life who can see that too. You don't need her, ultimately. If you can fix things with her, great. Now you can make up for lost time and your friendship could be even stronger than before. If not? Oh well, life goes on, you will continue, and most likely, you are probably better off.
