Help! BFN then BFP Possible?

13 dpo and my period is supposed to start tomorrow. My cycle has not been super regular since I just got my mirena out on February 20th. I thought I felt myself O around the 20th of this month. I have tested negative today, however I had sharp pains today (thought AF was coming, but no sign of her. Lots of milky discharge though). My question is has anyone ever gotten a negative test at 13 dpo and later a positive? Trying to decide if I should count myself out or just wait? This is so hard!