After I haven't eaten for hours and then eat, why do I feel warmth spread through my body?

well, if you are a vampire, this process is explained in detail in Interview With The Vampire by Anne Rice. If you are not a vampire, then I would suppose it could be any of the reasons you stated.

I think your body is thanking you for finally providing it with some sustenance. Food = good. Starvation = bad.

I definitely feel warm after I ingest warm food, and similarly get cold when I eat cold food. I actually have very little ability to regulate my body’s temperature, and in the winter I’ll be freezing no matter how much clothing I put on until I drink a hot cuppa something or other. I go through an obscene amount of tea in the winter, mainly to keep warm.

In your case, it may be your metabolism deciding it has enough energy to “kick the heat back on” and amp up your metabolism/blood flow etc, but I don’t know. It’s pretty cool though. :)

I can also feel it when alcohol hits my bloodstream, it’s weird.

It is normal to have a small increase in both core temperature and skin temperature when digesting food.