How can I fix this problem?

The pony I lease usually likes to kind of verge into the inside of the ring, but I can usually handle it just by using my inside leg, outside rein and he'll stay on the rail. But the past 2 times I've ridden him, for some reason he's been worse about it. He'll try to get to the inside and I do everything I can to keep him on the rail, ex. inside leg, outside rein, but instead of listening to me and staying on the rail he fights me instead, throws his head up, and refuses to get on the rail. I don't know why he's being so difficult all of a sudden. I really need to fix this, it's getting frustrating and i'm all out of ideas. What are your suggestions?