Why does my pony have the runs?!?

my mare is rising 5 years old and she's had the runs since early autumn (fall). she always has the same amount of haylege and she is exercised daily, so i don't think it is down to lack of routine. ive tried putting her on hay, but it makes no difference. ive tried feeding her bran, but it made no difference. ive tried no speedy-beat, but it made no difference. im at my Witt's end because i don't know what else to do because im going through more bedding that necessary plus it must be uncomfortable for her. she wormed and vaccinated and her coat is shiny and she doesn't seem stressed, she's just as happy as she always is. please help. thank you