When my mare did this, she was colicking. I couldn't get a hold of my regular vet, so I called the emergency vet and he recommended mineral oil. I'm afraid I can't remember the dose, but it was 2-3 bottles of the size you usually find at the drug store. Maybe pints? We used a turkey baster at the back of the throat like you would do a de-wormer. She had a bowel movement within a couple of hours.
If your horse hasn't had a bowel movement, or if you can't hear movement in her belly, that's a good "amateur" way to tell. Otherwise, maybe laminitis. I would call and speak with a vet...many will just give you advice over the phone for free, until you decide for sure whether to have one out. Regardless, lying down a lot isn't a typical behavior, so be on the look out. Good luck!