would it be really bad to call in sick on the second day of a temp job?

i really don't like the job, it its soul-draining and requires a lot of heavy lifting and yesterday i think injured my back a little so today i've woken up feeling really achy and also have a cold. i haven't slept properly in two days because of nerves so i also feel very lethargic. i just don't feel like i could hack todays shift! i would quit if i didn't need the money for christmas. would it be really bad if i called in sick today so i could recover and return all fresh and prepared for next week? if so, what would be a reasonable enough excuse?

p.s. the job is a xmas mail sorter for royal mail