What is wrong with my horse!?!?
Nervous during ride? If she seems like a nervous horse, she may have been mistreated during rides and is anticipating this. Or, perhaps the tack doesn't fit right, causing pain. Try gaining her confidence and acceptance with ground exercises such as grooming, leading, lunging. Make the sessions short, then put back in paddock or stall. Do be sure she gets outside with other horses, if she is used to them. Do you ride her near other horses or stalls? Are you riding in a ring or on a trail or the road? Consider that the conditions where you are riding can affect her, especially if it's something she's not used to our nervous with, such as riding on the road, or near a stallion or aggressive horse. Consult with a trainer. Here's a good forum, one response suggesting "hindgut ulcers". https://www.chronofhorse.com/forum/forum/discussion-forums/horse-care/241307-stress-diarrhea
Sammy, this horse has a nervous gut. She's going to have loose stools whenever she's ridden or does anything out of the ordinary. Some horses are like that, it's just their personality, and there's not a dammed thing that you or anyone else can do about it. Quit wasting your money chasing after shadows. The vet isn't going to be able to do anything about this- it's just the mare's personality that's causing her to do this, not a medical issue. We had a gelding once who had the same problem. The horse I RIDE also has a tendency to poop a lot when he's being trailered or ridden in a place he's not familiar with. It's not likely that your mare has ulcers. The fact that she grinds her teeth and acts skittish when she's ridden tells me that she's high strung and has a nervous personality, something which should have been apparent to you before you bought her. If it wasn't, then that's a pretty good sign that she was probably drugged when you test rode her, and it also means that you've been tricked into buying a horse that has some major personality problems. All the ulcer medication and vet visits in the world WON'T CHANGE a horse like this for the better, and you're WASTING YOUR MONEY if you continue to look at this as a medical problem. It's NOT a medical problem, the mare has a PERSONALITY PROBLEM. One other note- putting a horse on alfalfa when the animal isn't used to it is a recipe not only for diarrhea but also for COLIC. You're lucky the mare didn't get really SICK on you. We don't even feed alfalfa where I live, because one its expensive, and two, it's way too RICH. Grass hay is better, especially with a horse that is already high strung and temperamental the way this mare so obviously is.
Judy and the Beast
Some horses are just prone to loose poop all their life. Or it could be temporary nerves of settling in. If the vet is not concerned, I would not be either. Try consistency in her feed and training schedule and see if it evens out. Additionally, green grass and alfalfa are known to cause loose stool in horses. If that bothers you or the horse, stick to grassy hay. The fact that it resolves inbetween makes it seem like a behavioral issue and not from an infectious disease.
Another the nag may be she has sand in the intestine, have you checked for that? It can be an irritant and cause diarrhea from being on Timothy grass. Even exercise can stress the colon moving things around and reirritating if the horse is exercised, the gut bounces and moves more than if it is standing still causing the irritation to act up. Also if a horse has been on bad feed and is not used to alfalfa or good grasses and hay it could possibly cause loose stools and irregular bowels, but after a month or two it clears up. Horses that are stresses or are in pain may have this exciting slso, I agree with PR's post, check the tack, work with the horse by trying exercising on lunge, no tack to see if there is a problem afterwards, stress or pain can cause change in the bowels when a horse is pushed. Did you know the seller? Why did they sell their horse, was it for this reason perhaps? Just a thought. Good luck, keep working with your vet but also check to see if the horse may be eating something in its stall or paddock it may possibly be getting into. Also another thought is about the water supply. Fresh water to drinking out of creek, stream or pond?