Mare kicking and biting stomach?

And let's hope she's not the P word. There weren't any stallions around, right?

More than likely it's just bugs. Some horses seem to attract bugs (they seem to like our sorrel mares more than our white and bay ones) and some horses are much more sensitive to them. All the fly sprays in the world only help a little. In my experience for these horses, a nice dark shelter with a fan helps more than anything else during the prime bug time (around here it's from about 10am to 5pm when it's sunny and hot). When the bugs are really bad, our sorrel mare Mandy will run into her shelter from the pasture to escape them and stay there all afternoon.

The suggestions brought up are the most likely.If she has a bunch of bites & burs on her belly that can be irritating. I wouldn't discount possiblity of ulcers,they are more common than one thinks. The change of feed & new place would just be added stressors to agrivate things.Just a little video clip about ulcers..

If you apply pressure to the area, does she show any indication of pain that way? Can you isolate a specific area that is causing pain maybe by feeling around yourself? Did she start this as soon as you brought her home, or did it start several days later? Is there any edema/swelling anywhere? Possibly something in the fields that is toxic that is causing upset? Is she still drinking, urinating and defacating normally? Is she lethargic at all? Is her heart rate normal?

If you know the people you got her from, can you call them and ask if she ever did this while they had her?

I'd say if you can't come up with an answer, you need to call the vet and let them know what's going on jic you need them. The vet might have other ideas to check and monitor. THen you can set up a schedule for when you should have them come out -- right away, or give it another day or two. But, if anything changes, you need to be on the alert. "Nothing" can change to a serious something in a heartbeat!

I would make sure she is not preggers!!

Youre positive the mare wasn't bred when you got her? Could be that she is getting close to foaling...we have a mare on our farm that is due to foal any day and you would hardly know she is pregnant up until a few days ago as she is small of build and looks almost normal. She bites at her sides because the foal is moving. It looks almost like colic when a mare is due to foal.

EVERY domestic animal has problems changing feed, every time. Dogs can have bleeding intestines from sudden changes. Horses can colic.
We don't have any pictures, but ONLY gradually changing hay, and gradually getting used to the grass on your property will guarantee no impactions, no colic, and no diarrhea.
I suggest you put her on grass hay ONLY. If you're getting a lot of rain you WILL colic or founder her on really lush, fast-growing hay.
I'm not familiar with the ulcers above and their symptoms, but OBVIOUSLY she isn't comfortable. Have your Vet out.