skin around mouth and under nose doesn't tan?

sorryyyyy for the awkward picture 😂 for as long as i can remember, the skin around my mouth has never tanned. i don't tan in the tanning bed and when i'm laying out, i make sure that my face is at a good angle so that my nose doesn't cast a shadow. i'm very very pale in the winter but in the summer people question my genetics lol. i never burn. in the picture i had just gotten out of the sun and i was hot. i never ever wear foundation or any makeup on my face other than my eyes. i wear chapstick but i'm sure not to get it anywhere but my lips. does anyone know what the cause of this is? it's very annoying because of how awkward i look when i'm not smiling.