My dog got a knot two days ago. It has tten rather large I would say softball size,and it is tender to touch. It's located on the bottom part of his ribs. He still is eating and drinking. He is just moving a little slower. I have seen spider bites that has done this same thing, but never gotten this big. Does this sound unusal. The knot is hard to the touch.
A lump that large sounds almost like an abscess, especially when you say it's tender to the touch. I am assuming you mean that when you touch it he flinches. If it is hard, it isn't ready to lance if it is an abscess. I have seen dogs get swellings on their faces from spider bites but never that large. This could be a large hematoma, or a contusion from trauma. This is something the vet really needs to see as quickly as possible. The fact you state that he is 'moving a little slower" makes me think he could have even been bumped by a car. Check his toenails and inner thighs for abrasions, also. But in the mean time, call the vet.