Any ideas why my feet hurt when I ride?

I ride hunters/jumpers, and I've noticed recently that when I ride, my feet hurt in the stirrups. I think it's a pressure thing perhaps because when I ride bareback or without stirrups it doesn't happen. My boots are old (I'm getting new ones), but I've been riding in the same boots for years and saddle/stirrups for the past two years and the pain has only been recently. My heels are down, and I have not been injured. Does anyone else have experience with this problem, or ideas of how to fix it? Thanks for any and all input :)