Worried!

A lot of tmi,but....i had unprotected sex end of November,missed my period for the month of December,and have taken 3 tests,but they were negative. Or so i think. A second line appeared on my last one but the color was very faint. And a couple days ago i began getting brown discharge. And today started cramping and lower back pains. But no period yet. Should i take another test? Could this be pregnancy? I’m so worried. Please give any advice or opinions.