How does this work?

So I have a question that just occurred to me: when I’m engaged in work I love or doing something that’s fun to me (like taking an online course or reading a book and taking notes ) I get a tingly excited feeling in my stomach/body. I’m not having any conscious thought that make me feel excited or happy, but physically in my body I feel that sensation. It doesn’t happen all time time (like when I’m cleaning the litter box) but I’ve noticed it when I’m doing something I really enjoy. What triggers that feeling/sensation? And if a feeling is a sensation in our body, how is this triggered without a conscious thought?