Sandie's been doing this EXACT thing lately as well (and like you, I have also already ruled out soreness as an issue with her)...I've heard from many riders that it's the cold weather, it makes them crazy! I have just been taking extra time (putting it only on the first hole until after we're out in the arena) & warming up the bit just like Vida mentions (we have an old hair dryer in the barn that everyone uses to warm it up quickly -- just watch it's not HOT, don't wanna burn any horsey mouths!). Stick with it & have patience, and she'll get better! But I agree with you that if she were sore, I would think she'd be acting up in the ring as well, not just when you're putting the tack on.
You could try this though (if you haven't already and it sounds like you may already have): take your thumb and pointer finger and run them down her back (pressing FIRMLY) along the sides of her spine and see how she reacts. If she doesn't do anything, it's likely not a back issue. If she flinches or reacts like she does when you go to girth her, it just might be and you may want to call in a vet or someone who specializes to check it out.
I have my vet coming out on Monday to check on Sandie's eyes...I'm going to mention this exact scenario to her about Sandie since she has been doing this lately as well, so I'll let you know what she says about her!