False negatives?

I'm wondering if I am getting false negatives. I have PCOS so maybe that has something to do with it. I don't get normal periods. I actually haven't bleed in 6 months. But every month around the same time I get period like symptoms. My symptoms haven't come this month so I have missed a period. I am 18 dpo. But my cm is thick and creamy and has a slight odor. My boobs are tender and my nipples are very sore to the touch. I am so nauseous all the time and sensitive to foods and smells. I have had a headache for a week my back is killing me. I am so tired. I have never been this tired before in my life. I have gotten two negatives but I'm wondering since my hormones are not regulated that my hcg levels may not be high enough.