I've had a brownish black spot of the same size under my big toe for weeks. doesnt hurt at all. no other health problems. only 21. what could it be?

ABCDE's of Moles: It sounds like you have a mole. Moles are benign collections of melanocytes - the pigment producing cells of the skin. Normal moles develop in childhood, adolescence and early adult yrs. Regular borders, uniform color, size <6mm are considered normal. Asymmetry, border irregularity, color variation, diameter >6mm, and evolution are warning signs of melanoma. See a derm if any abcd occurs.

ABC's: Although i agree with all the posts previous, use the abc (for melanoma) to help relieve some stress before you see a doctor is the spot asymmetrical? Are borders irregular shaped? Is the color uniform or multicolor? What is the diameter? Also is it elevated? Lastly, of course get it check-ed out but those abc's;s should help relief some stress. Hope this helps!