I'm getting so pissed off at my job. I was told when I was hired that I wouldn't get less than 20…

I'm getting so pissed off at my job. I was told when I was hired that I wouldn't get less than 20 hours and I could easily pick up hours. well the next two weeks I work 19 hours and 16 hours and there are 0 shifts to pick up... how am I supposed to pay the rent now! I always get screwed over by my jobs!