Hi Rachel, that's a fairly serious concern - diarrhea which isn't diagnosed and rectified in short order can cause or be attributed to more serious and possibly life-threatening health issues. Not knowing anything about the horse's breed, health history, what's in your pasture, etc., my first suggestion to you would be to remove the horse from pasture right away if you possibly can, and keep him/her off until you can determine the cause. Horses have delicate systems, and diarrhea's a pretty serious and immediate concern. I hope you will be able to find out what's up.
If you've just got her, and have put her onto green pasture - that's probably your problem.
I would advise limiting her turnout time, or fencing smaller sections of her yard off and gradually increasing it until she becomes accustomed to that pasture. Also, provide ample meadow/grass hay, which will provide fibre and help firm up her manure.