You can have them check to make sure she's okay and then follow up with your regular vet on Monday. Many times the groups are affiliated, and the recheck on Monday could be at no charge. Won't hurt to ask about that. The fact that it popped up so quickly and is growing so fast means it is probably NOT a tumor. But even if a vet were here (and they aren't) he wouldn't be able to say what it is. I do know this much. She should have been taken to a vet this morning as soon as you noticed it was still spreading. It could be anything, including a snake. And if it's a lymph gland, there's some kind of major and serious infection spreading rapidly. If you're in the US, there's an emergency vet within driving distance, and I would have had her there this morning if regular vets weren't opened. And that's what you need to do now. There's just no other answer, and it sounds like she's worsening pretty quickly. PS - I just noticed you asked in the UK section, but it really doesn't change anything if you're in UK. She has got to be seen by a vet asap, whatever it takes. Nobody on here can answer with certainty on what is going on. You need to take her to the vet immediately before its to late! If it were your child you wouldn't wait to see what happens! They have emergency vet