Why it is so painful to be stepped on accident by high-heel shoes?

Very high pressure on your foot.

Explanation:

Now let's say this lady who stepped on your foot is 70 kg. The force she exerts is her weight which is about 700 N.

Now, if those 700 N are spread over a larger surface area, as in flat shoes, the pressure she'll exert on your foot will be much less than if her 700 N were concentrated on the tiny heel's surface area.

According to the formula,

P = F/A; where P is pressure in Pa, F is the force in N and A is the surface area in metre square;

Pressure is inversely proportional to area. The smaller the area the higher the pressure exerted. And that's the case with the heel.