We have some in barn that you can not tell, and then mares that for 2 weeks straight kick boards off, push on stall walls, and are wired for sound.
And there are no stallions there, nor anywhere in 20 miles or more.
During heat cycle, mares are more apt to be girthy, ill tempered, kick out, bite and strike.
And agree with whoever said that problems may have been there, but it suited her to go along with program too.
As for handling her out in open or riding? Be aware that a stallion will, if gets chance, attack mare and also the rider too. I've seen Shetland stallion years ago run across field, drop and ROLL under a fence and come up and attack several of us on mares during trail riding.
And not all mares are calm during heat cycle, some will be, some won't.