I'm 18 Days Late For My Period. I've Had Gas

I'm 18 days late for my period. I've had gas and slight cramps near my kidneys, backaches near my lower back/kidney area and when I wipe sometimes I find brown clumps and most recently white clumpy, cottage cheese like discharge. My tummy seems tight but it might just be in my head. I've had 4 negative hpts taken well after 2 weeks late. Could I be pregnant?