Trip to a good lameness vet. They will do a lameness exam, block the feet to make sure pain is in the foot, xrays. If xrays don't show anything then there is always MRI for soft tissue ideas. 

The good Skips Watch Jo Lady mare I had was cold backed. It was worse if it was cold, or windy. I messed up a few times. You had to saddle her and let her soak. When you got on, if she felt tight most of the time you could back her up a few times and she would loosen up. On a bad day, you might have to jog for 30 minutes before she got level. It didn't make much difference if she was in a full time program, or had been turned out for a month. She was a true 1D horse with several riders, from professionals to handy youth.

I have two suggestions. Check for ulcers and check teeth.  

My Hired Gun filly was pretty broncy to start even still if she hasent been ridden in a while she needs a good long warm up. 

She's not dirty, but holly hell is she loud, squels and honks like some of thoes bucking horses. I stay on and look awesome so whatever lol. 

I'm not sure she will be a top barrel horse shes my rope prospect but I'll pattern her anyway. Shes just 3 now.