could this be a back problem?

Theres this horse that I want to ride but she bucks and I've been working with her on the lunge line with no saddle or bridle on her and she's never bucked . I was thinking that I will try working with her saddle and bridle on on the lunge line and see if she bucks.. someone said maybe her back needs adjusted but she's never bucked around me before. I do know that she does not get ridden often and maybe that's why. Im going to talk to my trainer about it tomorrow. But what do you think I should do

Sounds like you have a saddle that doesn't fit! Make sure it is wide enough and doesn't cause pressure points that create discomfort which causes the bucking. I had the same thing happen with one of my guys. The only time he was cranky and bucked was in my saddle- on the lunge and when I rode. Borrow someone else's saddle that is wider and see what happens.

Sometimes when a horse is asked to canter it's off balance and scared and scared horses will buck. When you do your lunge work with her saddled, just do walk and trot work and 3-5 rounds each at the trot then the other way. Don't do any more than 5. If you ask her to canter you are inviting her to buck. Work her 5 days doing walk trot as horses do better with daily repetition. By now she should be doing nice walk to trot transitions, trot to walk and stopping. She is now used to wearing a saddle. Don't rush this process. After the 5th day coax her into the canter (no lung whip) for a few strides then slow to trot. Don't haze or yell at her to canter but try just making kissing or clicking noises. Keep doing the transitions adding one or two canter strides before the downward transition. If she does fairly well after cantering two or three times, cut the lesson short and put her away. This will pay off the next time you work with her.

Saddle fit might be the issue so make sure it fits her. I know I have a horse who will send you sailing if saddle is to narrow across the withers.Had this issue before could ride bare back no problems. Put saddle on and the rodeo started even on lunge line.She could also be out of wack. Id check the saddle fit if you don't know how too get someone who knows how.