There are several possible causes. One that should probably be investigated is the possibility of a granulosa cell tumour. This is an estrogen-secreting tumour of the ovary that will often start by manifesting persistent estrus, and then finally often change to stallion-like behaviour.
Not all mares "shut down", so don't assume that you will see that, although it is a possibility. When they do stop cycling though, they are more likely to appear in a condition like winter anestrus, where they are neither opposed nor overly receptive to a stallion - just a kind of "ho-hum" attitude, that is often mistaken for early estrus.
If she has a uterine infection, she may be short-cycling herself, and only being in diestrus for a matter of 8 days or so before returning to estrus, so if you are not seeing the same behaviour every day, that is also a possibility.
I would start by having her ultrasounded and evaluated for internal reproductive issues.
Thank you Jos. I will call my vet and schedule an appointment for an ultrasound.
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