Excessive heat due to Yang excess. These are purely TCM 中醫 terms, and the words vary from one book to another. Anyone not familiar with TCM won't understand this idea.

I think you can say 'get inflamed'...

西医没有这种理论 要是真的要说上火是 inner heat

i think you can say "hot" inside your body. But actually, lots of people have not concept of "上火" except chinese or some asian, so i'm not sure all people can understand. such as "It's too hot to eat too much pimiento." "you will feel bad if you get hot inside your body"