What Causes Dark Spot On Labia Minora After Sex?

i recently had sex for the first time, and about 2 days after, i noticed a dark spot on my labia minora.. as days passed it has been getting bigger. i never thought anything of it, but its been about 3 monthes now, and it kinda looks unnormal. i did some research and saw it could be cancer. which does run in my family.. or if it had something to do with my sex partner. im kinda getting worried.. help?