Metallic taste in mouth, thin red slightly itchy rash around mouth for over a month now. The metallic taste is new as of a wk... What is it?

Possibly. Candidiasis... A fungal infection.. See your physician or dentist for an evaluation and treatment.
Red rash. It could be a case of candida (fungal infection), it could also be due to an allergic reaction to something new in your diet. You are young but in women hormonal changes can also affect the mouth. The candida can also be a reaction to abx tx, so if you are on one that may also affect. Changing the bacterial content of your mouth lets other stuff take over. I would talk to your dentist.