I am so sorry.... it is a difficult decision.

When suffering is paramount..that is when to do "it" IMO.

We've put down 4 cats...one had cancer, two had a stroke, and one with kidney failure.

We just learned 2 days before our vacation (from which we have just returned) that Oreo -- our black and white has a tumor on her thyroid. Her blood tests were not elevated...so it is wait and see with her. I am taking her to a cat specialist very soon for another opinion. Not sure it is cancer, but she seems frail all of a sudden. Our Pooski had thyroid cancer 30 yrs ago...and I remember that painfully still today. Oreo seems to have improved a bit on vacation and is eating well again. I won't know until I get the second opinion what is in store for her really.

I really feel for you. Take it slowly and see what happens. If lymphoma in dogs is like some in humans, there can be a very slow progression. In people, it is called "indolent" and may be present for many years before even noticed or diagnosed. Feeling tired is basically the only sign, with the swellings. Not all lymphomas in humans are indolent, but some are.