am I freaking out to much ???

okay, i need a better understanding please. do you have to have a missed period for hcg to detect on pregnancy tests? Or how does it work? Because I took a test on Saturday and got a faint, but when i take these cheapies it’s negative.. im not due for my period til tomorrow. But ive been real nauseous here lately, and lastnight i had this rush of sharp pain run all the way down to my lower back.. IF i am pregnant why isn’t the test picking up?? Am I to early or???