I have had some, especially first time moms, become extremely protective, but never like this one of yours! And yes, usually after a few days, they settle down some.
If she is threatening, or actually attacking? What has been your reaction?? If you run away, this is teaching her that this behavior works. If her stall is fairly large, or there is a place where you have enough room, I would put a chair in the corner, and just sit in it. Don't try to invade her or the baby's space. Let her calm down and know your presence is not going to hurt anything. I would do that for several days til she is comfortable with it--- then I would try grooming HER, not the foal and never get between her and the foal either!
You will have to be patient and work with this one slowly, and usually first time moms are the worst, but this sounds pretty extreme to me.
I have had mares kick at me, or pin their ears and stick their teeth in my face and make a short lunge at me, but when I stood my ground- they didnt come any closer- they were just threatening. If this mare really means to attack, I hate to say it but as a last resort, carry a short riding crop or something to protect yourself with if you must.
I had a friend who bought a really nice gelding back when we were in school, for next to nothing - a full sized horse. He ran the entire family out of his pen. They would not even go in to catch him- threw his feed in over the fence - they were all terrified of him. It was a game to him and he thought it was funny. Of course, they didnt tell my friend this when she went to look at him to buy him- she went right in, walked right up to him- caught and rode him and bought him! He was always a gentleman and had loads of personality. She had him til he died at about 29 years old! We had more fun with him and he showed well for her too. Nothing he wouldnt do. He just had their number and they ran out when he made threats, which then escalated to running them out literally.
You know your mare though, and the gelding mentioned above didnt have hormones out of whack and a new foal, but SOME behavior, or continuation of it, is learned. Perhaps your vet may have some handy advice? I would avoid tranquilizing if at all possible, but this is a big hump in her behavior that she is going to have to get past. She will continue to do, what you will allow and tolerate.
Just BE CAREFUL and make sure you have 'eyes in the back of your head' on this one!! I would not remove her from the foal either to work with the baby- even my gentle mares would go ballistic doing that.