I wouldn't be overly concerned. Laying down twice in one day is not what I would call excessive. Generally, horses will only lay flat out when they feel totally relaxed and safe in their environment. Of course, a severely ill or lame horse will lay flat out and be unable to rise, but the chances are that your horse is enjoying a cat nap. If her mouth were really bothering her, I doubt she'd be lounging like a sun worshipper - she'd more than likely be standing up with a pained expression on her face.