You said three weeks ago. I guess you meant you were told three weeks ago that it happend 10 months ago?but i was told 3 weeks ago a colt got into the mare's field.
That does not look like an advanced pregnancy, and you already had your vet check, so I would say not pregnant. If you are still convinced, then have another vet check her. I HIGHLY doubt a vet could miss a foal that's near ready to be born though, they take up a lot of room in the mare!