Horse deciding to poop in stall?

I acquired my mare in November. I keep her at home on my own property. She has two stalls she can go in/out of at her discretion for shelter. Until two weeks ago she has never pooped in the stalls. For the past two weeks she has been pooping in the stalls. Nothing has changed regarding her lifestyle. Could... show more I acquired my mare in November. I keep her at home on my own property. She has two stalls she can go in/out of at her discretion for shelter. Until two weeks ago she has never pooped in the stalls. For the past two weeks she has been pooping in the stalls. Nothing has changed regarding her lifestyle. Could this have something to do with being in heat? FYI...she comes with me to my lesson every week, we go trail riding once a week, we do arena work at home 2-3 times/week. All of this has been consistent and nothing has changed.
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