It's actually quite a common injury with race horses. The tendon gets overextended because of the push for speed even when the horse is saying they are tired. As you know a horse will still continue running with a broken leg if they have to.
I am pretty sure there is no way you would have known what was happening because the gait would have been weird in the deep snow and not felt like it normally does anyway.
I had no idea what a bowed tendon was until I saw it in the horse at the trail stables. One of the grooms had taken the horse to the beach bareback and the horse had not been at our place long and so was not used to the sand. He rode him hard in the deep sections (I think because the horse was excited on the beach and he was bareback so there was less chance he would slide off). When horses go into deep footing it is almost impossible for them to go at any speed as it takes a lot of effort due to the weight of the drag created by the sand/mud. This is what strains the tendon.
We are very mindful of staying on the firmer sand nearer the water when we take a visiting horse with us on the beach. It is tempting for the owners to want to explore the dunes and ride fast in the deeper sand because they feel safer but it is extremely easy to injure the horse this way if he normally lives on grass pasture or firm ground.
The horse that had the bowed tendon was sound again about three weeks later. We hosed twice a day, let him rest for a week or so in a small paddock and then let him out to graze with the quieter horses. The vet said he was fine to be ridden as long as he was not made to pull anything (cart), go into deep footing or any kind of jumping (repetitive).
It was not a few weeks after he was back in the herd that the owner took him to the beach and galloped him. He came back with both front legs bowed and badly. I wanted to cry.
She thought she knew better than the vet and because he healed so quickly the first time she did not believe it to be serious. He is now retired in someones garden as a pet.
His legs will always have the bowed look and he can't be ridden. This horse was about 14 at the time and was one of the best horses temperament wise you could wish for. This is another reason why they get it. The good natured horses don't protest or buck/loose the rider, they just find that extra ummph and give it regardless and the damage is done.
So I guess you won't be riding in deep snow any more and now you know to stay clear of heavy footing. They call it a bowed tendon because it looks just like a "bow" in shape and I think you just stopped in time as it looks like you only have the swelling at the base and only a little. So I guess it's an early warning and now you know what could happen and how to avoid it.
I am sure in a week or two the inflammation/swelling will have gone down and you won't see it anymore just be careful she doesn't charge around in the next few weeks or rear up in play and come down heavy on it. Like humans most sprains or strains take about 6 weeks to heal fully even though they feel better after a week or two and it's tempting to resume normal activities.
P.S Quiet movement (where she can choose to move or not) will help it heal as the blood flow will aid in the healing process. Hosing with cold water will help too.
If you know anyone in the racing industry near you, they will be knowledgable about this type of injury and may be able to tell you more precisely by looking/feeling exactly what is going on and advise you accordingly.