I just started walking fast on a treadmill and now my foot hurts on the bottom. On the bone that's closest to the big toe. What do you think this is?

Sesmoiditis. Could be sesmoidits under the great toe joint or you could have hallux limitus or the great toe joint. See podiatrist for exam and x/ray.
Could be many things. Including not limited to sesamoiditis, hallux limitis, plantar fascitis, tendonitis, etc.....Try icing, maybe an arch support and if no relief see your foot doc.