Thanks so much for the answers to my questions.
That's good that gum color is normal; it should be pale pink so perhaps this is what you're seeing. It's also good that there's no gastrointestinal distress or discomfort when you palpate his abdomen.
I understand about taking his temperature but it might be quite helpful if you could do so. A fever could explain his symptoms of lethargy, panting and trembling (although there are other reasons which I'll include below).
Normal temperature is between 100 and 102.5. If elevated, then we'd have to wonder why, of course. The possibilities are numerous including tick diseases, bacterial or viral infections, auto-immune disease to name a few reasons. Most, if not all, of these conditions typically develop quite suddenly with no warning.
Other possible explanations for his symptoms could include:
1. He could have pain/discomfort somewhere in his body...hips, knees, spine, for example, despite the inability to localize a specific area of pain.
If he is not currently taking any nsaid or steroid medication and since he's not vomiting, you might consider giving Aspirin at a dose of 10 mg/lb twice daily, with food to avoid stomach upset.
Aspirin has anti-fever properties as well as anti-pain and anti-inflammatory ones and might be a good choice in this situation.
My only hesitation about its use would be that it might interfere with what your vet would want to prescribe if he needs to be seen but several doses should be fine.
2. As strange as it sounds, I have seen some dogs with anal glands behave as you describe. If you are not familiar with these glands, this LINK discusses them in detail.
Some dogs are extremely bothered when these glands fill up which causes pressure on their bodies and will continue to do so until they are emptied. Although a groomer can provide this service for you, they usually attempt to empty the glands externally. The better method is done rectally which is how a vet tech or your vet would empty them.
3. If he ingested a large number of apples, then he might be experiencing gas pains although 6 hours is a fairly long time for him to be uncomfortable. But, if this is a possibility, then anti-gas products (which usually contain Simethicone) such as we would take for ourselves can safely be given. The dose is somewhat empiric but usually about 1/4th-1/2 of what we would take.
4. Of course, anxiety/fear/stress can also cause trembling or panting but seems unlikely in this case if there haven't been any thunderstorms or fireworks to trigger this response.
I'm sorry that I can't be more specific as to the underlying cause of his behavior but as you can see, there are several possibilities. If you're able to take his temperature and it's normal, then a several reasons can be ruled out (specifically all of those included in #1). But if it's high, then a vet visit might be prudent although sometimes it's a challenge to determine the cause of a fever if a physical exam is normal.