Mazda 3 brake problem

Good Day!!
i have some problems about our mazda 3 brakes, kc everytime na malulubak ako or sudden brakes bigla nlng lalalim yung brake pedal nya, kaya medyo kakabahan ako kasi medyo hihina yung brakes nya, pero after ko ipump ung brake pedal babalik sya sa dati, ano kaya yung problems nya s brakes?