The lump is like half a baseball it feels attached and hard

I don't have a clock withe a second hand but I can put my ear on the lump and loudly hear her heartbeat through it as opposed to other areas. I would say 2-3 beats per second. Seems very fast.

Breathing seems okay. while laying down but was panting loudly on walk ( it is hot and humid) but the panting was almost immediate.

It seemed to suddenly appear but It is 1/2 under her arm so It is hard to say but I tend to think it is very recent. Probably past day or two.

She has skin tags and at one point had a huge one the size of a index finger the vet said it was a giant skin tag that didn't require surgery. (no biopsy) It was soft and squishy. She ended up chewing it off.( The dog not the vet ) just trying to lighten the crisis.

Nothing that ever looked like this. It looks like a giant cyst or tumor.

I just started her on a new brand of hip and joint formula about 3 days ago. It seems to coincide with the change in her demeanor. might just be coinsidence.

She doesn't get much exercise lays on the couch alot while i'm at work and home.

There is no discharge or pimple head just a big lump. Thanks John

Thank You, XXXXX XXXXX that you can't give a hands on diagnoses on the internet but if it is a cyst / tumor / abcess are they usually easily treatable ? I know each diagnosis is different. I guess what i'm asking is is this common ? Is the outcome more positive or negative when it comes to my dogs chances? Are all tumors cancer? Thank You for your pending answers and the answers you have already given. I'm starting to feel a bit better. I will take her to the vet tomorrow. It's not easy to love a dog for 12 years and discover something like that.