Help! Bony lump on Fetlock!

[swright]

My 5yo TB mare has developed a lump on the inside of her hind fetlock. It has some fluid on top but is quite hard! Dont think it's a windgall due to hardness of it but its too low to be a splint!

Theres no heat or major swelling and she is very very mildly lame on hard ground but not sure as I was running with her and trying to look at the same time! She isnt sensative to the touch though.

Does anybody have any ideas what it could be?

 

[Charem]

Could be a windgall, my lad had a very hard one on the outside of his NF just above fetlock. Was sound for roughly 7 months with it and then he was very slightly lame. Scan revealed his tendon sheath was inflamed.