Expecting mini horse..Why is she so big?

We have purchased a wild miniature horse herd, and one of the mares is expecting. We haven't gotten them yet but we are worried about her. She is like huge and her bag is also huge. We keep expecting her to have her baby any day but we are now concerned she might be pregnant with twins. What are the chances that she is? Or could she just be carrying the baby longer than normal? I mean her bag is like the size of a goats when its nursing. I've never seen a mares bag that big, its practically hanging between her legs. She also looks like she is uncomfortable walking because of the weight. We can't get a vet out to see her, what should we do?