Learn Portuguese (Portugal): Comi muito bolo, por isso agora dói-me a barriga. - I ate a lot of cake, so now my belly hurts.

Literally meaning "for that" or "because of that", por isso is the most common way to link a cause (Comi muito bolo) to its consequence or result (agora dói-me a barriga). While então can sometimes mean "so" (like "so, what now?"), por isso is the exact connector you need when you mean "therefore" or "for that reason".

Why does the sentence say dói-me a barriga instead of "a minha barriga dói"?

In Portuguese, it sounds unnatural to use possessives (like minha) with body parts. Instead, you use the definite article (a, meaning "the") for the body part, and add an indirect object pronoun (me, meaning "to me") to the verb. Literally, you are saying "the belly hurts to me".

Since me means "to me", what is the actual subject of the verb dói?

The subject is a barriga (the belly)! Because it is a singular noun, the verb takes the singular "it" form (dói). If you had eaten so much that multiple body parts hurt, like your teeth (os dentes), the verb would change to the plural form: doem-me os dentes.

Does muito change depending on the food I eat?