Please take your Mom to get a second opinion. It is not possible to know what caused the bite, but the problem is that the result is getting worse and not better. The swelling shows that the bite has caused an allergic reaction. If the pus was present, I would think it was a staph infection. Even though it might not be a staph infection, the fact that it is getting bigger could mean that more and more of her skin, tissue, muscle, could suffer from irreversible damage. If she is feeling poorly that could mean an infection is spreading that the current course of antibiotics is not healing. Please get a second opinion. Your Mom deserves a second look. Best of luck
Use salt on the wound to draw the poison out and have her drink milk. We get scorpion stings here all the time and that's what we do. First wipe the wound with vinegar, then put a compress with salt water on it and tape it on. it should draw out the venom and she should start getting better. So sorry this happened to your Mom.