Mare- possibly carrying a foal, but shows no signs.?

she was bred in august of 2006 and we think she is pregnant, but she is not as big as she should be, and she has been having discharge, but it is most definitely not her going into heat. she has milk, but she has also been a "surrogate mother" to a colt who refuses to stop nursing. she has started to force him away to wean him, and he is old enough to be weaned.[1 year and 6 months old]. she is starting to want to be alone out in the field, and has a sucky attitude sometimes. she is somewhat ertive toward the other horses, and her attitude has changed completely since she returned from being bred.