A few days ago Roslyn wa

s playing with Oona and let out a huge yelp and hid. I checked her out thinking she probably got snagged by a tooth or something, but didn't find any marks. Since then she had been limping a bit, and very snippy with the other dogs. She has been super clingy to me and distant to everyone else.
I figured she had pulled a muscle or something as there was nothing external and she wasn't having any other symptoms beside being grumpy and clingy.
This morning I went about my normal routine of checking her out (I like to reassure myself often when one of my beasts are acting strangely) and found a HUGE mass had developed over night right in the middle of her chest! Ok, maybe not massive... but it is at least the size of an egg and developed over night.
I am scared to death right now. I know that most masses are cysts and nothing to be concerned about... but with the pain and the size and the speed I am near tears.
I have called my vet and they will be looking at her at 11:30am... but I am doing everything in my power to keep it together. (thank you mycorgi.com for providing my with an outlet for my stress. If I posted on facebook my family would roll their eyes and think im crazy for caring this much... as she is "just a dog".)

Being a nurse I have 3 scenarios running in my head.
Harmless cyst/tumor
Hernia
Harmful tumor

Am I missing anything? Could it have something to do with the pulmonary system? Am I being a complete and utter headcase for nothing?

I guess in the back of my head I am always worried about cancers because we adopted her when she was 5yr old (she is 7yrs old now) and had her spayed then... I have read that cancer is more prevalent in dogs that are not spayed or neutered early in life, but I am not sure HOW much more prevalent (there wouldn't be very many breeders if all there females died at 7 yrs old...)

Thanks for listening ... I feel much better "talking" to someone before the vet appointment. If I said all this to my (non medical) husband I would just worry him as well. You all are a valued friends to me!