Your mare is getting very close to foaling. Look to see if the foal is laying on one side or the other? If you look at the mare and one side is bigger and sticking out more then the foal is laying on that side and is getting positioned for birth. Your mares belly is big but it could be dropped down more. Sometimes however a maiden doesn't show a big drop. I would say that she could drop more. Her vulva looks pretty elongated which means she's close. You said that her butt is very jelloey, push all along her tailbone and along the top muscles of her butt. The softer they are, the closer your are to foaling. This gets very soft and gushy within a few days of foaling.
Lastly, check her utter. Although some mares, especially maidens won't get big utters, most get some build up. Look to see how full her utter is and if it's big and hard. Is there any "wax" or whitish driplets on the ends of the teats. Can you milk or squeeze a few drops from the utters in your hand? If you can, is the milk yellow or white and is it sticky or not? The stickier it is the, the closer you are to foaling. It will get very sticky and start to turn whitish when you get really close, like within 24-48 hours.
If you see all these signs, dropped vulva, soft tail head, sticky whitish milk, then you'd better not leave her side because the minute you do, she'll foal
I hope you've read up on how foals are presented and different positions and what's an emergency. Most foals are presented properly and most mares foal fairly quickly. You might want to help it along a bit by helping the feet and head out by pulling gently on the feet while the mare has a contraction. Only pull when she's contracting and only gently. Make sure to pull the plancenta off the foals nose and clean it out as soon as it appears so it starts breatheing. Finally, don't forget the enema and putting betadine on the stump. Good Luck, you're going to have a newborn in a few days.