What could this be?

For the last few days my mare has had a funny lump on her leg. It is on the inside of her hind leg and is where a splint would be. There is no heat, she is not lame at all and it feels like a hard bony lump. There was a cut there caused by a knock in the field which is healing.

What could it be and should I be worried?

My YO thought it was an old splint but I'm sure after a year I would have noticed it, I'm 99% certain it wasn't there this time last week!

She is very prone to knocking herself and although we don't work on hard ground she is an absolute nut in the field.

Umm... if no one has any ideas I'll take pics tomorrow.
 
Sounds like a splint to me too. My mare formed one on the outside of her hind leg (caused by a kick), she was never lame on it either.
 
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