She may need her teeth done.
If her teeth are fine then doing a lot of bend and flex work should help.
Of course she could bracing against you and being stiff because she is not liking the bit or other pain issues.
I would keep and eye on her and take it easy, slow work and really make sure she is placing her body in the correct position for turn....you know correct leads and feet placement...make sure she isn't favorong anything or dropping her shoulder or anything like that.
If all is ruled out then I would just work on the basics and give it time....she may have just been having a bad/off day.