It sounds like she is outgrowing the saddle due to changing body shape. Also sounds like she's a bit girth sour. Possibly from pinching. Check your girth for flaws and don't try to tighten the girth all in one go.
How old is she that you are running her, but she is still growing? Usually the two don't go hand in hand like that... Or maybe she's done growing up and is just filling out now? However, if she is young and downhill, she might not be done growing in her bones yet.
I tighten my girth snug, then piddle around doing other stuff for a few minutes. Then I snug it up again, stretch out the front legs and tighten the girth until I can get my fingers under, but that's all. Usually a few minutes of walking will loosen it up again, so it's worth another check. Now for my purposes, that's generally good enough, but for competitive work you'll probably have to check it after a 10 minute warm-up.