The Vets I have known tell me these transitional periods are colic season. Some horses seem to forget they need to drink when the weather changes, and next thing you know, there they go. I've seen everything from just lying down and sorry for themselves ("my tummy hurts") to diving at the ground when you try to walk them ("why don't you just go away and let me die?"). Thank the Lord, I've never yet seen the agony of a twisted bowel (and pray I never do!!). If you like, I could point you to a couple of websites where you can learn more than you ever want to know about colic - as could several other horse owners here, I'll bet. Good job, and good luck!