Second resonant frequency of a microstrip antenna

I've been designing a microstrip antenna which is radiated at 2.4 Ghz,

but somehow from the simulation result(see image below), my antenna also radiates at around 4.6 Ghz and 5.2 Ghz.

I need some theory to explain about the second frequency and why does it occur.

Please help!

For the microstrip antenna, its impedance behaves like a shunt RLC tank at the first resonance freq, while it resembles a series RLC circuit at the second resonance freq. As a rough estimation, the second resonance freq is about twice of the first one. With respect to your case, where there are two troughs around 4.8 GHz (=2*2.4), the general rule of the second resonance freq still holds. The two-trough phenomenon is mostly due to the subtle variation of the impedance value around 4.8 GHz.

That means the microstrip antenna has two characteristic impedance?

The plot appears quite strange to me as well.

By microstrip antenna, do you mean a simple microstrip (rectangular/square) patch antenna ? Does it have any extensions/slots or other modifications ?

Most of the resonators have multiple resonant frequencies which occurs at multiples of the fundamental wavelength.
The amplitude of these parasitic resonances sometimes are harmful for the resonator circuits which can be antennas, filters, etc., so you have to minimize this behavior if the resonator (antenna) was designed as monoband.
Basically you have to think that your circuit lose radiated energy at those unwanted resonances, decreasing the efficiency of the antenna at fundamental frequency.

I press on what jeeudr said "Does it have any extensions/slots or other modifications ?"
make sure of that first, then go for justifications of your results

Patch can be best analyzed using the cavity model. The lowest order resonance in patch represents the fundamental model. The other resonances which you are getting are most probably representing the higher order modes. Consult the Balanis book on Antenna Theory and using the dimensions of your antenna, find the resonant frequency of the higher order modes and compare them with your simulation results. Also, look at the radiation efficiency of your antenna at the higher order resonances. Most probably, the will be very less compared to the efficiency at the fundamental mode.