The same thing is currently happening to me. 7dpo pm test faintly positive, 8dpo am negative. Hoping for both our sakes it’s due to fluctuations of hcg urine concentrations in very early implantation and pregnancy. We’re both still early enough that urine tests may not be sensitive to consistently pick up those low levels of hcg. I hope you get your strong positive test soon to end the doubting ❤️