Do occasional periods of immobility signal potential heart issues for a Cavalier King Charles Spaniel?

I have a ruby-colored Cavalier King Charles Spaniel who is nearly 8 and a half years old (born March 2012). I read online that a large fraction of Cavalier’s suffer from heart murmur, which typically had early onset and ultimately leads to congestive heart failure. The resources I found typically stated that the onset of such early symptoms begins around 5-6 years old, and progresses over the next 3-5 years. I am wondering if the symptoms my dog is experiencing may be the early onset of the common Cavalier heart problems.

My dog had first been experiencing some things one year ago; we took him to the veterinarian and they believed that he was having issues related to swollen glands, which were giving him the following symptoms (which occurred in this order):

1. Following whoever was nearest in the house around, acting like he was fearful and needed protection.
2. He is typically a very dominating type of dog, and growls especially when others faces are placed near him. Don’t misinterpret this; he is affectionate, just doesn’t like being kissed on the head, for example. During these symptoms, however, he doesn’t seem to care, and doesn’t really grrrr at all.
3. Periods of slow movement and no desire the be physically active. Also unable to jump off or onto the couch (which is above him, but this was never a problem). Not moving off the couch often (looks up at the couch because he just wants to sit and relax his body, but doesn’t want to try to jump up as normal, even when prompted to).
4. Not going to the bathroom, but eating as normal without any problems (contradictory to the hungerless symptom of Cavalier heart issues)
5. Being resistant to being picked up, like something hurts. Occasionally yelping when moved, but with no visible injuries, rather, it appears he is yelping out of fear of pain or something.

3, 4, and 5 come together.

After a few days, and a vet visit, he was fine for about a year. Then, a few weeks ago, the same occurred. We made an appointment with the vet, but this time, we were a bit late to notice and act on the symptoms. Surprisingly, however, the issues appeared to resolve automatically even before the appointment (in about 3 days). This was odd as it made me think they were entirely unrelated to what the vets had hypothesized last year. 3 days later, we wake up, and the dog immediately goes to the bathroom, and wags his tail and runs around constantly, behaving the total opposite. It seems like the dog knew something was wrong and now knows he feels better.

However, I’m still worried, for these brief issues are really impacting the behavior of my dog for the brief period they occur and they seem quite abnormal. I’m also worried these may be signs of heart issues do to the predominant symptom of inactivity my dog is experiencing during this.