I had a mare who had a baby a couple years before I got her, and she had bigger teats than a lot of mares I've seen that haven't had babies before, and she did on and off secrete a clearish liquid when I would clean them. But not knowing her history, I'd definitely get it checked out just to make sure. I've known of a few mares that have come in really emaciated, and sick, and have ended up being pregnant, and the babies came out perfectly healthy and normal, so don't worry too much about that if she is pregnant. Just focus on getting her healthy again. Let us know how it goes, and what the vet finds.