Am I pregnant?

I took a test after being a week late and they both came out negative. The following week my boobs became very sore (especially my nipples) and I am having cramps/ back pain - basically period symptoms. I had a yeast infection about a week ago and now it's been 18 days since my missed period. Today I went to use the bathroom and wiped and there was a little bit of blood, so little that the color was pink. (I've also been having creamy discharge.) I'm freaking out, I thought after taking the tests and getting a negative I could finally relax. Does this happen often? Are the tests unreliable?