Well, now I am slightly upset because I went ahead and read about bladder cancer.
She hasn't been straining to urinate, no blood in urine but it takes her a very long time. She doesn't mark her territory anymore on walks so this may suggest it is uncomfortable. There are two sac like lumps on the inside of her left legs which makes me think enlarged lymph nodes and this is the leg that suddenly went lame. I read that the cancer can spread to the urethra and eventually block it or to the bone and cause lameness.
I know I have no diagnosis here but I certainly feel worried. I will get a complete blood count if that will show something. Her slowing down and "old dog breathing" which is how the vet described it, could be all related now.
She just had a blood panel done last Spring and everything looked good with the exception of slightly elevated pancreatic enzymes but nothing serious. I really hope it's not this but now I am thinking about it more and more. I always wish I could have gone back and been more observant or really took my gut instincts seriously that something was wrong. I have actually been thinking something was wrong for awhile but have been trying to reassure myself that it is just old age.