Hematoma?

A day or two after I moved Kwik to our new barn, I noticed a little swollen area on his left hindquarter above the stifle area, probably about the size of a quarter. It honestly just looked like he got bit by something (unfortunately we still have mosquitoes here from time to time since this winter has been so mild). It got slightly bigger the following week, and I pointed it out to the chiropractor. He thought it looked like a bite as well and said I could treat it with witch hazel. There was really no change last weekend, and it rained the majority of the week so I didn't make it out until Saturday. When I pulled his blanket off, I met with THIS:

It's about 6 inches long and filled with fluid- feels like a water balloon. You can see it jiggle when he moves (gross), but he didn't appear to be bothered by it at all. He was not off, and after consulting with the BO and a few of the boarders, we decided it was probably a hematoma and a weekend vet call was not necessary.

We went for a short, light ride (he was absolutely amazing, mad-crazy wind and all!), and he was his same perky self afterwards. Naturally, I raced home to research hematomas. It definitely sounds like what Kwik has going on is presenting like a hematoma, and while I read it can take months for them to resolve, the body usually reabsorbs them on its own and the horse is usually not physically affected. I did read that in some cases, they have to be drained, but most sources say that is not recommended initially. I read several times treatment with DMSO/furazone is helpful, but I'm going to send some pics to my vet to get her recommendation. In the meantime, I'm thinking I should let Kwik take it easy. Bummer! We have a lesson scheduled with Anne on my Monday day off :-(
Anyone have experience with something like this?

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