I agree get a second opinion or a blood test or an ultrasound. If you are worming every 6-8 weeks maybe get a worm count done as the worms could become resistant to the wormer.
On a slightly different note my mare was showing from 3 1/2 months in and she was a maiden mare.
This was her at 4 1/2 months she wasn't btw.
But this is the size of foal she had and the stallion was only an inch bigger then here.