Good Morning and welcome to Just answer. I am Dr. Loretta, a licensed veterinarian and I am happy to answer your question.
It is not unusual for the mammary gland to come up and down before delivery because the oxytocin has not yet been let down since she has not delivered yet. The gland is getting stimulation from the hormones to prepare and this hardness is probably the gland preparing itself. This would be especially true if this is her first. There is no set way that the mammary gland will prepare itself and tis is different in every mare. There is always a chance that his hardness could be caused by a mastitis but you will not know this until she delivers this foal.
Mares are often overdue and if she appears normal, I would not be concerned, I have seen mammary glands do different things prior to delivery with no bad side effects.
Sincerely, XXXXX XXXXX