Resonance and Q LC tank

I have a PCB loop to which i attached a capacitor. I couple this resonator to 2 pickup loops and connect these loops to the network analyzer and measure S21

I indeed see a high resonance peak, but just to the left there is a strong negative peak. Any suggestions what causes this ? Is this normal ?

I think it's normal. Just from the spectrum shape, but i don't know what you are doing and its schematics.