I have a small pea sized lump on my left labia majora near the top just below the level of the clitoris- it is immobile and doesn't hurt. what's this?

Retention cyst: If you are sexually active, then you better see your PCP for examination . Many STDs can present with a small bump. If no history of sex contact then this may be a simple retention cyst.

Epidermal Cyst?: Hard to say w/out doing an exam or a picture of the lump itself. My guess is that because you're young and likely shaving/waxing around that region that this could be what's called an inclusion cyst where a gland/follicle is clogged and your body creates a cyst around it; it can also be an ingrown hair. Not a problem if it doesn't grow/cause pain. Try hot compresses to shrink it. See MD otherwise.