Don't know if your mare has foaled, hope these tips can help.
When she is close, she will become very loose on the rump either side of her tail like jelly, she is just relaxing her muscles, preparing to deliver.
she will also look as if she suddenly lost a lot of weight, it will be hard to tell, as your mare is so under weight in the first place.
This happens to mares that are due to deliver, I am suprised the vet didn't tell you this.
The virginal area will look smooth (not all wrinked up) and enlarged,
I would also suggest when they are due that you be there, things can go wrong, (yes sleep deprivation is going to happen) the joys of having mare in foal.