Of course I cannot reply for your mare but Gauda, my little mare, was on heat very regularly when she was still at the farm, as of 1yr old. On the other side of the road stood a stallion (Flemish horse - the very big ones), so I guess there were lots of hormones in the air.
Also the other mares were on heat every 3 weeks.
Since she arrived with my little herd of 3 geldings (1st of May this year, she was good 2yrs old than) she has never been on heat again.
I thought it was because there were little male hormones in the air now, but still I got worried some time ago that she might be carrying - I know the farmer wanted to give her a foal even though she was still very young but I did not want that.
She was quite skinny for a long time and I could not get her a bit fatter on the ribs. But her tummy was sometimes sort of hanging out on the sides (I hope you understand what I mean?), so I wondered if she was going to have a foal after all.
In the meantime she wears her tummy in a normal way again and her ribs are covered a bit better.
Now this may also be because she has got some winter fur already, but I have the impression when I touch her that she put on some weight, finally.
(She is lowest in ranking -also at the farm-, so she had difficulties getting her food properly. So when I noticed this I started with 3-4 different hay spots, so she could eat a lot better without being disturbed all the time.)
So I don't think (and certainly hope so) that she is not carrying. I want her to grow adult first and have babies later.
But who knows, I did not have a vet looking at her... I might still be in for a surprise!
I know this is a bit off-topic, but does anyone have more experience with mares not being on heat for so long? She is my first one, so I have no other experience...