possibly pregnant mare

Question
Her belly is extremely large, even though she is very under-weight.  The vet is coming out next monday.  I've also noticed that since she's at my barn she mingles fine with the other mares, but when my gelding comes to check her out she pins her ears straight back and has a major hissy fit.  About her eating.  She's eating the food I give her in a matter of seconds.  She gets about 3 lbs twice a day.  I'm just afraid to give her too much too soon considering all she's had is round bails in the pasture.  Thanks for the info.  
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The text above is a follow-up to ...

-----Question-----
Hi I'm buying an approx. 14 y/o TB/QH mare who may be pregnant.  The dealer I'm buying her from said she's been a broodmare for years, and has been pastured with a stud for over a year and was recently sold to him so there's a possibility she may be pregnant.  What I'm wondering is if there is any way to find out if she is, or signs that she is.  She's also very underweight, and if she is pregnant how will this affect the foal.
-----Answer-----
Dear Geri,
        If she was in a pasture with a stud she most likely is pregnant, but to be sure look for enlarging belly, crankiness, and more eating. To be sure you want to call a vet because if she is underweight when she is pregnant she could possibly die or loose the foal.
Michelyn

Answer
Dear Geri,
        She probably is pregnant but be very careful how much you feed her. A underweight horse that eats alot like that may colic, try to give her all her food but in smaller portions. Like feed her in the morning, then if you can bring her in the stable s few hours later and give her more. Feed smaller portions but the same amount through the day. If she does colic while she is pregnant she and the foal could die. Be extremely careful and see if you can't get any pregnant mare grain for her. If she doesn't have the vitamins she needs feeding her foal off her milk will drain her.
Michelyn