I have the same thing, no one wants to ultrasound a mini. I think you have a better chance than I do that yours is in foal, fingers crossed for both of us though, the babies are WONDERFUL. I had one 3 years ago(the same one I'm waiting to see with right now) who didn't show at all until 3 weeks before she foaled. Don't stress about her not being big, they don't all look it. If you get a picture about every week especially of her udder it might give you a better comparison. Keep feeling every day in case there is a big change but she looks like you have time yet if she is. The one I had last year(a rescue that was maybe in foal) had me up every 2 hours doing foal watch for 4 months. I didn't know when she was due and she looked a lot closer than 4 months away. Try not to panic, read everything you can and get a foaling kit together in case. She'll let you know when she's getting closer.