This is normal with mares - they are just going back to their "wild" protective instincts. Our mare foaled in March and for the first day or so we could do anything,t hen the hormones kicked in and we were barely able to enter her stable and muck out. This lasted for a couple of weeks when she calmed down considerably (we had been warned by the vet that this could happen even with the gentlest of mares) and we able to give her baby a scratch and a rub.
Now, he's 3 months old and she keeps a watch on him from afar unless there's food around then she's first to us lol. But we can do pretty much anything we like with him now and she trusts us and knows we're not going to hurt her precious foal!
We weren't experienced enough to hobble her or force the situation, but we had some good advice and plenty of willing helpers when she had clamed enough for us to start his "training". He's now able to be led round the field, we can spray him with insect repellant, lift his feet and groom him and worming's no problem at all! He's too **** nosey for his own good but we gave her a bit of time and that worked for us. Maybe ask your vet for advice too and they may have a different answer.
Good luck and be careful - a mare will go to any lengths to protect her baby xx