Just thought I'd add my 2 cents...
The first thing that comes to my mind is pain, which has been mentioned many times, then there's the lazy/spoiled mentality, but the thing I haven't seen on here yet is... is it possible she could be pregnant? We had a mini I rode when I was little. Me, my brother, and my sister rode her all the time. She would ride with or with our a saddle, with or without a bridle. Very easy, do what ever you asked of her. SHe was bought to be a brood mare, but found to have scaring from a foaling wiht a previous owner, so she became the horse us kids played with and showed. She got turned out with stallions as a companion from time to time. ONe summer me and my sister got her out to ride her, and she was very mean and nasty, didn't want us on her and would do what ever she could to prevent us from getting on, or to get us off. Being kids at the time (and this was 16 years ago) we chalkedit up to a bad mood, but her "bad mood" continued for a month or 2.... until my dad went out to feed one day and found a filly running by the mare's side! After foaling she was back to her kid friendly self.
Just a thought as it sounds similar to your situation, except I don't know if you even own a stallion.