Freaking out!!!

Today we noticed that my kitten Hopper has a huge lump on her neck. It looks like the size a large walnut and is in her lower jaw. She wouldn't let me open her mouth and has been very inactive. I'm trying to get a hold of the vet to find out costs to get her looked at...but in the meantime I'm freaking out. I'm going to be a wreck until we know what it is. It just appeared overnight and I can find nothing on-line that can offer and consolation. My sweet baby. It is going to be a long night.