A really weird lump

[Equestriangirl123]

So for the past few days my pony has had a swelling on the inside of hind leg just below the hock. The swelling is on his big main vein and inside it is a very small pea sized ball lump. He's fine with me touching it but if I dig my nail in slightly to the little lump he finds it a little sore .
There is no heat just fluid .
Any ideas ?

 

[SatansLittleHelper]

Maybe he has knocked it?? What has the vet said?? X

 

[hol089]

If it's just appeared suddenly could possibly be a haematoma if he's knocked it or been kicked.