Theres a black dot underneath my foot. i believe i got it when i stepped on a pebble, and it hurt for a while. it doesn't hurt now but the dot is still there. i'm wobdering what is it?

From what you described it is likely to be a blood clot under the skin. It should resolve in a few weeks. If it does not resolve, gets bigger, ulcerates, develops a nodule or other spots develop, see a doctor.

Without examination it could be something or it could not be anything. The “spot” could be hematoma. This should be absorbed and disappear within 3 weeks. . If persistent I would get this examined by a dermatologist.