Under What Circumstances Does A Slot Antenna Change from single band to dual or

triple band frequency shift?

I changed the length of the slot and it changed from single to dual band and I dont know why.

A slot antenna is a dual structure to a dipole. It has therefore a similar radiation pattern. Like dipoles, slots are often considered to be resonant antennas, with an optimum length equal to 0.47 wavelength. But in general, dipoles and slots can radiate at other frequencies while their impedance and radiation pattern changes.
If you observe situation like described, check also the matching circuit that feeds the slot from a transmission line. They cannot be separated in a real situation.