ouch!! I don't have any experience with bunions - hopefully someone will have a good suggestion about what will work for you! Good luck!!
as for why? hmmmm Could anything else be "out"? - like your hips, knees, etc... could your saddle have gotten out of whack? is there a crack in the sole of your shoe? could anything about your riding position have changed? maybe something is slightly different for some reason and that's enough to torment your tender tootsies!
I hope you find the cause and enough comfort to do your ride!
namaste,
~T~