Its probably best to check her back for pain too (a vet wll be able to give a clearer diagnosis that yourself) as she could have twisted it or bumped it playing or rolling in the field and its stiff and sore to move- hence why she no longer canters off and is reluctant to be as 'hyper' as before. The vet may give bute if they think it is pain, but wait for a vets diagnosis before giving any medication yourself.
Did you get a pre purchase vet exam before you got her? Did anything minor show up there which may be shining through now?
She's beautiful btw