I think it's a matter of style, I would find either acceptable, but I think capitalization in this context might be more common. Capitalization is used, I think, because "Chemistry" is a subject or course title, and not just chemistry in general. You might, perhaps informally, be studying chemistry in general, in which case, I would definitely not use a capital letter, but in the case of a "major" it is a formal course.
Capitalization is much discussed and debated, if you put the word into the Dictionary Look-Up box, you will find many threads on the subject in various contexts.