What does this sound like to you?
The last few times I've ridden my horse he has tripped a couple of times during the ride, which is unusual for him, so a little red flag went up in my head. Rode him in a silly/fun/games type show at the barn on Sat and he seemed sound, but my husband, who always seems to be right about these things, asked me what was wrong with him (the horse) because he didn't look right. (Not soundness-wise; horse was just standing on x-ties.) Trainer rode Sunday and said he was really good. Then in my lesson yesterday, as soon as I started trotting he felt slightly off. It felt like left front. He worked out of that, but just never felt like himself. He felt like he was shuffling behind, especially around corners. I normally have trouble sitting his trot, and I had no problem at all sitting the trot--another red flag to me that he wasn't moving quite right, although trainer said he looked good from the ground. After my ride as I was leading him out, he suddenly seemed to slip badly even though there was nothing to slip or trip on--almost like he locked a stifle, which he has never done before.
The weirdness, vagueness, variety, and transience of these issues makes me think of Lyme. ???