Horse has colic symptoms - only when exercising
I'm leasing a 10-year old mare who has always (for two years at least) had some problems with pooping while being ridden, but the problem has gotten worse recently. She sometimes suddenly stops while cantering and puts herself in peeing position without peeing. When she feels like she's going to poop, she gets very tense, lifts her tail up and refuses to move. I have to force her to keep on trotting (otherwise she won't poop), but she gets very annoyed, swishes her tail and seems very uncomfortable. Previously it took just a minute or two for her to finally poop, so this wasn't as big of a problem. Nowadays it can take 15 minutes. After she finally poops she's forward and nice to ride, until she starts the same thing all over again. This happens up to three times during a lesson, and happens also when I lunge her. It's getting to the point where it's impossible to compete her, and very hard to train her. She has no colic symptoms in the stable or outside, and seems to have no problems pooping there. Her poop looks normal, and she seems to be otherwise perfectly healthy.
She's getting 8kg haylage per day (we feed her 5 times a day), and 1 kilo protein pellets (mostly soy protein) divided onto three separate occasions. She maintains a normal weight. We are giving her psyllium (worked very well for a month or so, but stopped working), more water (she drinks poorly so we soak the pellets in lots of water), linseed, and mineral oil. Nothing seems to be working. We thought at first that it's due to her eating mud outside, because there's some grass growing in the mud that she nibbles on all day long. That's why we tried psyllium, which actually helped in the beginning, but doesn't seem to do so anymore.
Has anyone else experienced this, and is there anything we can do? Our vet doesn't understand what's going on either.