Lump (abscess?) on hind foot

On Saturday evening, I suddenly noticed a lump on Dylan's left back foot! It is big pea size and doesn't seem to bother him at all. I have booked him in to see Mark tomorrow morning... I wonder whether it is an abcess again as he had an abscess on his hock a couple of years ago. But that was quite umcomfortable, so I am not sure... better getting it checked out.