What was her exercise regimen before you bought her? Perhaps she isn’t really fit yet for what you do, assuming she wasn’t getting much exercise? Some horses (both of my mares in particular) think poles on the ground are snakes or some other evil thing. You could try just leading her over them, spaced fairly far apart. Most horses pull their hind legs forward when we lift them, and some are more willing than others to extend. That might be related or not. If I were you, I would work her for a shorter time to see if it helps. Since she is turned out, I doubt it is stiffness from not being able to move around at will.
It is never the horse's fault