Probably a bit of both, and also it isn't going directly onto any of the surfaces of your mouth, it is getting churned up with the cheese and 2 bits.of bread before it hits you.
I used to like mixing marmite and mustard together in cheese and onion toasties (with ham or bacon sometimes).
I also really like cheese and chutney toasties.
Did you butter the outside? Did you grill or sandwich press?