I have a TB gelding and I had same issues. I was feeding Alfa a oil completion mix and Top spec all in one supplement. I was advised by a judge out showing to just stick to my Alfa a oil, go on to Top Spec balancer pellets and sugar beat. I also lunge him twice a week now for half hour each time...
I find I get this with my horses rug. When I feel inside he's just right. It's only with his weatherbeta that this happends. My friend finds this happends with hers to. Just let others on yard awear as I had someone.... A livery Fresh from riding school strip my boy down to a very light fleece...