I agree with jr, she sounds colicy to me also.
I have rescued many, many horses, and this is what I do.
First off, get her seen by a vet. Have her teeth checked and floated if necessary. DO get a palpation done. It's not that expensive, ($20-$25) and that way you will know what you are dealing with instead of guessing. The vet may be able to tell you aprox how far along she is, if she is pregnant. She doesn't look
pregnant to me, but, I have one broodmare who doesn't look pregnant at all until about a week before she foals.
Your mare definitely isn't "bagged up". The vet can also do a fecal test to tell you how many and what type of worms she has. Normally it is not a matter of IF your horse has worms (most do no matter what) but HOW MANY and WHAT TYPE of worms she has. That way you will know what type of wormer to use. Often I will tube worm a new rescue. You can also put her on a daily wormer. But that is sometimes hard on thier systems if they are already down. Ask your vet what he/she recommends.
Jr is also right in the fact of slowly is the key. Buy the best feed you can afford for her. Get her on at least 12% protein feed, IMO 14% is better. I like a 14/6 mix. (14% protein, 6% fat) I prefer pellets, but that is a personal preference. Since she was emancipated, you want to build muscle
not just fat. Protein builds protein. (muscle) S-l-o-w-l-y increase her feed. Feed in as many small portions as you can. Do at least 2 feedings a day, 3-4 is better when they are really down.
She also needs to be on free choice hay. (if she already isn't) Meaning as much quality
hay you can find. A nice round bale is good. They will stand there and eat all day. If you can only get squares, then start with a bale in the am, and if she's eaten at least 1/2 of it by bedtime give her atleast another 1/2 bale. A flake or two of alfalfa hay (lots of protein) is good too.
Be sure to put her on a vitamin regimen. Especially
if she is pregnant. You can discontinue the vitamins (if you want) when she is at a proper weight.
I would add some probiotics to her feed. It is a microorganism that helps them to digest their food better and to "take in" as much nutrients as possible from the feed. Since she was so down, she probably has lost a lot of the microorganisms normally found in the gut.
Soaked beet pulp is also good to add to her feed. It is roughage, so you don't have to be concerned with over feeding her that. Make sure that she is fed the beetpulp immediately after soaking. Soaked beet pulp sours quickly if not eaten. I usually add about a scoop of dry to a bucket, soak it with water then add the grain to it.
Rice bran and corn oil is another additive you can try to increase her calories. Many horses don't like corn oil at first, so don't add too much at a time.
Personally, I wouldn't be riding her yet. A) it is painful for the horse if they are thin to carry a rider. B) she doesn't have the energy yet to carry a rider C) the energy she is expending riding is burning up all (most of) the calories you are trying to get into her. Be patient. In a couple of months she should be good to go.
Whew, that was an awful lot of information!
I hope it has helped some. You can pm me if you want/need more details.