What can cause horrible leg pain after working out?

Many things...but: Commonly stress fracture cause pain during or after working out. Usually this can be ruled out with a x-ray. More scary thinks would be bone tumors, blood clots, claudication ( angina of the leg muscles). Less scary stuff would be arthritis, muscle tears and strains, cartilage tears. My advice is see your primary care doctor.

Dehydration: You may need to hydrate yourself during work out , with electrolytes!, need good stretching.