Large odd hard lump

[mogs22]

My mare has a rock hard lump on her stifle area that suddenly appeared a month ago, she is not lame on it nor was any of the skin broken. Vet has ex-rayed and scanned and is still saying 'not seen anything like that before' which is not exactly helpful. The lump is not attached to a bone and look small when she is standing still but pokes out and looks disgusting when she puts her leg back in walk/trot etc.
Vet said to ignore which I had been doing and see if it goes on its own - which is the theory I had been working on, however it pulls on lots of muscles and as I am getting her fit I want to make sure I am not doing any damage.
Any one seen anything like this before - could it be a heamatoma?
Will get photo today and post later
Thanks

 

[brightmount]

Posting a pic below of a haematoma, does it look like that? As you can probably work out, this one came from a kick, you can still see the hoof print. It took several months to go, and got more pronounced before it started to go down.