Weird Lump..

So the other day I found a weird lump on Grace's flank, I put pressure on it to see if it was bothering her, it didn't so I pressed a little harder, and it slipped from under my fingers! I found I could move it anywhere I really wanted it is currently sitting behind the hip bone, so it can be seen easily. It is hard so I thought it might have been a bone chip, but she is not lame whatsoever. I have asked everyone at my barn if they knew what it was, and no one had seen anything like it.

Can anyone enlighten me to what it might be?
I am going to call the vet tomorrow, however I don't believe it is an emergency because she's not in any pain.

It almost sounds like a foreign body if it is not attached to anything. I would call the vet and see what they say.