Why does my horse have runny poo?

I have looked after a 20yr old warmblood mare for the past 3 years or so, and recently I have discovered she has runny poo.

There have been no dietry changes at all, she gets fed the same type of food but we have recently cut down the amount and she is on pasture. She often steals the other horses hay, which is just grass hay and she's never had a problem with that before.

The only change is that we have brought her back into work in the last two weeks, and I'm curious whether the change has somehow affected her digestive system?

We started working her slowly, nothing drastic, just 10-15 minutes of lunging or riding once a day, and a feed every 2 or 3 days, which is about the same as before.

Would the recent change of exercise be a cause of her runny poo?

I have never noticed it before though.

Oh and she is probably due for a worm, which I will get on to right away. I just want to know if the exercise and changes in her lifestyle could make her poo runny.

Thanks, hope someone can help! =]