I just had my girl spayed in March and that lump looks exactly like hers did. I called the vet and was told it's the swelling from the internal stitches. Dixie also pulled the top two of her four stitches within days of her surgery and I was told as long as the wound isn't pulled open, it'll be fine.

That's a healthy crust on your kitty so I wouldn't worry too much. If you spot some dried blood or seeping from it, then you might want to place a call but I think that looks like it's healing great!