I have an 8 year old mare who has had a sore back in the past. It hasn't been sore in a while, but the other day riding her she dropped her head down and stopped a lot, and was reluctant to go. When I asked for anything faster than a walk she would kick out and stop. She has done stuff like this when I've ridden other times, sometimes just a couple times then she acts normal.
She hasn't had much training in the past, and is in the process of re-training. She acts nasty and does weird things sometimes, so I'm not sure if she's in pain or just doesn't want to work.