Jessica Lanford you basically describe what it does! Although it showed up after he fell down out in the field
Sounds like a windpuff to me. Have had many horses who have had them, no trouble with them, and will inflame after exercise and get smaller while resting but never go away. There is no issue with them. Just a little fluid between joints from what I’ve been told.
Windpuffs, can also be a sign of arithritis
Please get a vet if you are worried about something like that. Seeking advice over the internet is dangerous and can lead to causing more damage. You don't know who is giving the advice and what qualifications they have to give that advice. For something like what you are asking, you'd likely have to be standing next to the horse anyway.
Sounds like wind puffs. Typically not a very big deal, many horses get them.