If it is soft to the touch, and you can kind of move it around, it is most likely just a cyst that could be removed but would be likely to come back. It may even get bigger.
Though we can not jump to a conclusion only by looking at limps but still if lumps and bump like this keep on increasing in size showing irregularity in shape then there are chances of tumors and cancerous cells. Their perfect diagnoses is only possible by biopsy tests are taken. This test requires a small part of the infected area to test. If cancerous cells are present there then radio therapy and chemotherapy is used to treat it.
If its she,she could have cancer
My female dog has a large mass from her under belly .begins in her hind legs and reaches to her front legs ..the width of her body,, she is 18 yrs old
My female dog just had puppies 10 weeks ago and she also has a cyst on her left side it moves around if you touch it she doesn't seem like she is acting different (behavior). What does the vet do to remove it and is it expensive.
You probably need to take her to your vet. It could be lymphomia or just a fatty tumor. If it grows that is not a good sign. Let the vet take a sample of the fluid in it and they will be able to tell you what it is.
Physical examination of the lump is necessary because it can be accumulation of fluid due to injury. Another possibility is cyst. For details, visit this link.