The vet came and looked. No obvious heat or swelling anywhere in the feet or legs. No strong digital pulses - they were pretty normal. Sometimes the lameness looked like left front, sometimes a bit on the right front at the walk and trot. Looked very much like left front when turned to the left, not so much when turned to the right. I rode her two days ago and she was sound - she could have stepped on a stone. She was sound leading out yesterday morning. I didn't lead her in last night but had no reports of unsoundness. She isn't really all that reluctant to walk - not at all how she was when she had her concussion laminitis last summer. She has very thin soles, as was shown on the x-rays we took last summer. With hoof testers, it seemed like the heel area in the left front was sore, and the inside heel of the right front, but no specific spots that suggest an abscess. After that, it looked more clearly like she was lame in the left front, and the soreness on the right may just be a result of standing with more weight on the right to take weight off the left.
The vet doesn't think it's due to the grass. She suspects that Maisie tweaked some structure inside her left front, either when I rode two days ago or while playing in the pasture. She can go out to graze for an hour in the morning and then another hour in the afternoon (I guess I'm not done with having to go to the barn several times a day), preferably not when hard playing will occur. Bute morning and evening today and tomorrow and once a day after that, and precautionary soaking of the left front, and then we'll see how she does, better or worse.
Of course when I turned her out, all the mares came running to the gate to greet her and running and playing ensued - no harm done that I could see - she wasn't completely sound but seemed comfortable playing (even though she wasn't supposed to).
Who knows what's going on? I was planning to ride today, but that's not happening for at least the next several days. I'm off to the barn again to bring her in - she's not going to think much of that!