Am I being a hypochondriac or is this just a new phase of development?
Hi! Sounds like you have done everything I would have done...chatted with the dr and are tuned it to see if anything else changes in a major way. I say keep doing that. If you notice a change in sleep/eating/more irritability, take him in right away. I know that my son has definitely become more focused in his play. He turned 4 in June and he can now sit and play with his trains/legos/hot wheels for an hour by himself without needing anything or stopping. He has bursts of energy, but it's not the same as when he was curious and into everything. I hear him being much more creative and talking to himself as he's using his imagination with his toys. Such a fun age! Good luck.
I think if I were you and touch base with the doc and insist he be seen again, and if the doc says it's nothing ask for detailed rationale. Although--I've had a gland like that on my neck (and a hard bump in my mouth) that are both apparently nothing--with the personality change and with the poop I would check it out further. It might be totally unrelated to the bump though. I've noticed DS is always calmer right before he gets strep!