how do you respond when people say youlook sad/mad when youre not?

i've started a new job recently and twice within 3 days ive been told to cheer up or asked what was wrong when i was just existing.

a) is this a common experience for y'all?

b) how do i respond? both times it took me by surprise and i just said "haha, my face just looks like that. i'm fine!"

the first time i thought it was a typical old white man comment (this job is on the blue collar side and many of my coworkers are old white dudes) but the second time it was from a fellow woman of color XD