Have you had a professional look at her? I don't know you, so maybe you are a professional, but I'm talking about having a chiropractor out, or a saddle fitter out specifically. Just throwing that out there, because I knew my saddle didn't fit my horse, and I've learned and been shown a lot about saddle fit, and I learned a whole bunch I didn't know, and couldn't see when I had my chiropractor check her out.
Just a couple of things to think about while you are up there, and when you think about what you've done the past few weeks, are you up off her back? When you trot are you leaning forward in a half seat position? Are your hands forward, or do you accidentally pull them back (even a little bit, not necessarily actually pulling on the mouth)? Everyone does things a bit differently, and every horse learns differently, but one of the things I've learned and been taught, is that initially you don't want to be sitting deep in the saddle, or be posting when getting the horse to trot, because it throws them off, and even when sitting deep in the saddle, you still bounce, and that has the same effect as posting.
Another thing I would try, cause it may well just be that she's decided to throw a tantrum as all children do at some point, have someone lunge her on the ground while you are on her back, and get her to trot, provided that she does lunge well.