You might not notice her cycle and they could still be bothersome to her.
When I worked for an Arabian breeder in college, one young mare, I think 2 year old, but it's been some time, came into heat during a show, it hit her so hard and fast she literally locked up in her class and wouldn't move. It took a lot to convince her to leave the ring. That was the end of her show day. Others are much more subtle, but no one realized this filly was coming into season, until she locked up in class. [It's been a long time, this is all the details I remember. I just vividly remember them saying it was her cycle that caused the problems.]