Chevrolet Cavalier Forum : “1999 Chevy Cavalier”

---Short Answer---

alternator  supplies the electrical power for the car.  As amperage draws increase the alternator has to compensate...  To do that the alternator becomes harder to turn.

-----Long Answer---

The alternator holds a relatively constant output voltage(v), so as more current(i) is used, the power(p) needed to be generated increases. p=v*i

Now power is work per an amount of time (w/t) and for a rotating source of work (w=torque*(theta)) theta/an amount of time is a rotation rate ( some RPM which is related to your engine speed)  So P=torque* (a rotation speed)

So if you turn more electrical devices on and maintain engine speed then your current (i) increases (P=v*i) so power increases and since P= also equals the torque times the rotational speed , and rotational speed is constant, the required torque must increase for enough power to be supplied.
(Note: there are efficiencies and reductions involved that make this harder, but the basics above give the relationship)

It could be many of things, might want to have someone look at it, ur pully could be to loose or your belt could be freying, i would honestly get a new belt and have it put on and see if it continues. because if your belt is bad or your pully is loose, then it will be to loose around your air conditioning/heating part of it.