I'm sorry to hear that you have concerns about Lily now - it's just tragic that our wonderful breed have all these problems and we have to expect that we are going to have to deal with many of these issues especially if we have multiple Cavaliers.
I think it's really helpful that people are posting their stories about their own experiences of chronic pain and how invisible it is to everyone else - we all try to hide pain, we don't want others around us to see us suffering - as you may remember this time last year I was suffering with a severe flare up of my Trigeminal Neuralgia, which is like having electric shocks applied to my face, it is described as the worst pain in the world. Only those really close to me could see it though.
In the wild, animals have to disguise pain and hide any weakness and that can't be bred out. It has been shown that people with a positive attitude cope better with pain and with chronic disease.
As you've read before in previous threads Dr. Oyama mentioned out of all the dogs The Cavaliers are the Happiest Dogs in the World! probably because of the high levels of Serotonin.
"Healthy CKCS dogs had significantly higher serum 5HT [which is serotonin] concentrations than other healthy dogs predisposed to DMVD." Serum Serotonin Concentrations in Dogs with Degenerative Mitral Valve Disease. Arndt JW, Reynolds CA, Singletary GE, Connolly JM, Levy RJ, Oyama MA. J Vet Intern Med. Nov/Dec 2009;23(6) 1208-1213.
"Interestingly, Cavalier King Charles Spaniels, which are highly predisposed to DMVD as well as macrothrombocytosis, had significantly higher serum 5HT concentrations than did other breeds of dogs." Insights into Serotonin Signaling Mechanisms Associated with Canine Degenerative Mitral Valve Disease. M.A. Oyama and R.J. Levy. J Vet Intern Med Jan/Feb 2010;24:27–36.
ANY unexplained pain needs to be investigated - dogs do not yelp for no reason.